There were some steel frames that used internal routing during the early 80s. Not sure about the 70s other than Klein.

To me the cutoff for Classic & Vintage is the Trek buyout of Klein, 9-speed, carbon forks become prevalent, and the point where the "steel is real" cult started to push back with their narrative.

I love that era.

Mavic SSC 8-speed indexing group which Sean Kelly won Tour stages on and Lemond won a tour on. Plus TWO different Mavic attempts, Zap and then Mektronic, looooooong before Shimano and Campagnolo dip a toe in the water of electrically powered derailleurs. Mavic astoundingly showing that their SSC road group was light enough for the tour but strong enough to be used as an off road group with essentially almost no changes whatsoever. Hubs had longer 135mm axles installed and everyone realized Mavic hubs were the same axle diameter as Phil Wood, only necking down narrower at the bearings. One of the only dependable freewheel 8-speed mountain bike hubs. Everyone else was bending axles.

Sachs New Success before the SRAM holdings became a tour de force in terms of market share. Campagnolo Ergolevers that were branded Sachs, but still had Campy hoods, they didn't even try to hide it. Gorgeous Huret made derailleurs before SRAM became a market player with all their acquisitions. The opposite of the Suntour story below.

Campagnolo Off Road components.

The working brand Suntour in which so many small companies had come together to deliver brilliant components in a consortium. I've never been exactly sure who was "in" and who was out but I've seen mentioned Ukai for rims, Sugino for cranks, Dia Compe for brakes, Sansin for Hubs (not sure about Suzue), and Maeda for the freewheels. SunTour themselves made the derailleurs and shifters I think. Missed the boat on indexed shifting and their very well respected groups like XC Pro and Superbe Pro were incredible in terms of fit & finish (Suntour XC Pro or Superbe Pro pedals are a thing to behold, WTB bought the "GreaseGuard" patent or leases it or something to that effect). Losing all the market share sucked for the other companies in the consortium. Tom Ritchey recognized how talented Sugino was and continued to use them for his Ritchey cranks for years. I had some nice Ritchey/Sugino 180mm cranks that were spec'd on my Cannondale M2000. Some of the nicest cranks I've ever ridden on. I love the Sugino AT touring cranks on my ST800s but at 175mm they are just too small to be usable by anyone riding a 27" touring frame. The opposite of the SRAM acquisition strategy and consortium success. Most people never think their SRAM derailleurs are really Huret, nor do they remember their Nissan was once called Datsun.

Funky Cannondale full-suspension bikes way ahead of the curve.

Bontrager selling to Trek, who immediately ceased production of his distinctive Race, Race Light and Privateer mountain bikes that were considered epic by many.

Lemond selling to Trek, who pretty escalated into Lance Armstrong bullying Trek and John Burke (head of Trek) into trying to bully Lemond. At this point the public didn't realize what in insufferable personality Armstrong was, who threatened to destroy Lemond's bicycle affiliation with Trek for the crime of having correctly and accurately having discussed Lance's doping. What Trek put Lemond through during this era is essentially disgusting. When people ask me what kind of bike to buy, the one consistent thing I share with them is that they should NEVER consider a Trek. That Trek made the morally repugnant decision to double down on their relationship with a caustic, abusive, bully who was deeply tied to their marketing efforts for the Trek brand and sales, and instead decided to destroy their Lemond bikes. Lemond is revered in France, but late in the game Trek wasn't selling but $10,000 worth of Lemond bikes (between five and ten) a year. Thousands of customers in France wanted to buy Lemond bikes, but Trek refused to sell them. The relationship fell apart with litigation on both sides. Trek disinvited Lemond from dealer shows and charity rides. Behind the scenes Trek was sullying Lemond's reputation in the industry. Later attempts to silence Lemond's criticism of Armstrong and former Trek/USPS rider Landis go to the absurd when after Lemond shares details of having been sexually abused, Landis tries to intimidate Lemond into not testifying by having an associate call Lemond threatening that his abuser will be at the hearing:

LeMond drops bombshell at Landis hearing - VeloNews.com

I still maintain that position. I strongly encourage people to avoid buying a Trek. I consider them a morally repugnant company. I'd sooner buy an Armstrong bike than a Trek. I'm serious. I understand Lance's passion and attempts to conceal his doping, the disingenuous cover-ups and the complicity. No one wants to get caught. However, for Trek what they did with their business decisions is deplorable and reprehensible. Deliberately destroying the Lemond brand in an attempt to create a chilling effect in the public forum by a great US cycling champion is just morally bankrupt.

The end of Zeus. The Zeus/Campagnolo relationship will never be fully revealed, I believe ever. However, no company pushed the boundaries of using titanium in components like Zeus did. Sometimes strangely. Zeus was a decade ahead of Campy using alloy freewheels. Zeus is considered to have "copied" Campagnolo derailleurs but many narratives say the relationship between the two companies was much more complicated. What's myth and what is just legend? Who knows. Was Zeus really just copying Campy designs, or as many claim was Zeus actually manufacturing and providing Campagnolo with all their titanium components to help Campy catch up? Zeus embraced drillium at the production level, which is just awesome. Zeus led the way in their use of titanium in their components. Mavic may have used a Titanium bolt on the old erector set 801/851 derailleurs, but Zeus was making everything out of titanium. Some would catch up with titanium spindles on their pedals, but few knew that Zeus had long been using titanium spindles AND cages on their pedals. Zeus wasn't just a "copy" of Campy stuff as many believed. The problem was they sometimes would use titanium throughout a derailleur and on the same derailleur use steel instead of titanium or aluminum alloy. It made no sense. The derailleur would resultingly be not much lighter than non-titanium Campy derailleurs. Go figure. At some point you could actually get a Zeus frame that was covered completely with Zeus bits, a head of Mavic and the 'tout Mavic' concept. What we'll probably never get is the whole truth on just how much Campy titanium components were actually being manufactured by Zeus.

Bikepro.com

Crappy plasticky mountain bike groups like 200GS, 300LX, 400LX etc. (which is ironic considering that modern SRAM groups are completely plasticky like my X9 setup). How is it all these years later I'm still riding crappy plasticky components?

Magnesium coming into the consciousness of cycling in terms of its potential for providing an unknown bound of lightness, stiffness, strength, vibration dampening, and durability, then facing from the consciousness of the cycling manufacturing industrial complex for being too unprofitable of a material to work with. Still the frame material of the future, always just the future. I'm still waiting. What Paketa makes great magnesium bikes out of can be compared to stovepipe of the steel era. We haven't even begun to glimpse the potential of what this metal can be extruded to be. Frame material pulled from seawater. Lighter than oversize aluminum by another 35%, eliminating steel from the conversation. 50% stiffer than oversize aluminum, 150% stiffer than steel AND offering better vibration dampening than any metal or alloy known. Think a plusher ride than the finest steel frame with the pure efficiency of a Crit Cannondale or Klein Quantum. Redfining the strength of a bicycle frame. Frank Kirk tried:

Cozy Beehive: The 8 Second Bicycle
Simon Haydn Online v2.0 - Kirk - History

Kirk used to drive his car over his frames to prove the point of their strength. I once backed my Cannondale ST800 into the garage. The back of the saddle hit the garage and passed all the force through the frame and it was the steel fork that bent. That is staggering to me just how strong Cannondale forks are. I still ride that bike, after I had the fork straightened. However, even I know that if I deliberately drove one of my cars, let alone a heavy Mercedes like Kirk used, over any frame material I ride I'd ruin the frame.

I'm still waiting for magnesium bikes to come back in any meaningful way.

However, all that being said of my favorite era that is still considered C&V, I would think my answer to the OP's question of what most modern vintage frame to build upon would be a Klein. My answer will always be a Klein. I don't think you can trust a Kirk frame to not fall apart, but if you could I'd say the magnesium bike. Absent that I say Klein. I love Cannodnale bikes and even my beloved Sport Touring bikes with their relaxed geometry and ability to carry absurdly stupid heavy loads, but in which you can remove the racks, panniers and touring bits and then still, in a pinch, race the snot out of them, even these STs take a back seat to a Klein.

Klein just has paint and cache that few bikes every match. Nothing, in my mind, matches the performance, rarity and scarcity, beauty and innovation as a Klein. However, as a one bike answer to build on my answer is almost always the same the Cannondale ST. Lighter, faster, stronger than almost anything you'll ever find. Titanium and Carbon can be lighter, but not stiffer AND stronger. Steel bikes can be built absurdly stiff, oversize thin wall steel in the dimensions that Klein and Cannondale used would be essentially unrideable due to the stiffness In cycling when we talk about strength we aren't talking about tensile strength, but the effective strength of the finished bicycle frame. As a material steel has a better tensile strength. However, a finished Cannondale/Klein aluminum frame ends up being stronger than a similar racing steel frame because of the density of steel means you get less "bang for your buck" out of the steel. Conversely if we built aluminum frames using the tubing gauges and dimensions that steel bikes are typically made out of the aluminum bikes would be so weak that frame failures would make aluminum frames essentially irrelevant. There is more to the story than just the metal, the metal and the engineering (oversized tubes or small tubing) is the context.

So for a I can only have one vintage bike to build on, and to have that do everything (loaded touring, gravel grinder, road race bike, commuter) there really is only one choice the Cannondale ST series. I say ST800:



However, if we are talking a bike in a stable of bikes, that just has to do one thing, if that one thing is go very very fast I say vintage Klein Quantum. Internal cable routing, takes the Cannondale "rocket bike" to another level and has a cache and coolness that few bikes can match. I'd have chosen wacky vintage carbon but that stuff is going to get someone killed someday, or at least that's my fear. I don't trust bikes built out of epoxy.

It I couldn't choose a US Aluminum paradigm changer the answer would always be Olmo. A Campy equipped pantagraphed Olmo. More history and credibility than essentially any brand other than Bianchi. He raced in the Olympics in LA in the thirties. Accomplished professional cyclist with Olympic Gold, 20 Giro stage wins, two time Milan-San Remo winner, National Champion, and even held the hour record for a bit. Olmo started Olmo Biciclette in 1938. Colnago isn't founded until 1952. Cinelli isn't founded until after World War II, in 1948. Masi starts making frames in the 50's. Pinarello in 1952. Other than Schwinn in 1895, Bianchi in 1885, if you want authentic and pedigree its Olmo.

Olmo is hard choice because for whatever reason while Colnago focused their marketing on the US, Masi focused on the US to the point they actually moved production to the US and then sold the trademark to US investors when Faleiro moved back to Italy, Olmo essentially has never bothered to care about the US market. Finding an Olmo in the US is very hard. They are almost always exquisite, dripping with Campy and pantagraphed. An independent company did import Olmos, but we know Cinelli, Colnago and Pinarello because they marketed heavily to us. The orthodontist and the anesthesiologist wanted one of those to "keep up" with members at their club. Few people even know what an Olmo is.

However with a vintage Olmo or a Cannondale ST those bikes come built with great components. To build a bike, maybe a titanium Lemond Victoire. Or Team-Z color scheme Lemond Maillot Jaune. Take that Trek! and shove it where the son don't shine.