I bought a 1984 Miyata NineTwelve. Stripped her down to just the frame/fork,headset,stem,seatpost and put on

Shimano 105: shifters, rear derailleur, crankset. (8 speed)
Shimano 600 FD, Ultegra Dual-pivot brake calipers and Ultegra Cassette (probably 2 years old at this point)
Crankbro's Eggbeater pedals
Mavic xp 22 rear wheel (old and beat up as heck) and Alex (generic front) (both 32 hole)
Michelin Lithion tires
REI RX saddle (not that impressed)

every time i rode on my girlfriend's 92 Allez with thin wall tubing i've been mad jealous about her ride quality. I thought my miyata would be comparable. I learned something yesterday I wish I would have realized a year ago. Having crappy old wheels is not the way to enjoy a great steel bicycle.

I bought a new Mavic Aksium wheelset. My miyata was reborn from the ashes.

So on to my question. What next? I'm thinking a new cassette is in order. Is it worth my time and money to upgrade stem/seatpost to carbon? Any recommendations for an awesome saddle ? (i know we all have different preferences. I'm looking for something that will be conscious of sensitive arteries and comfortable for 100+ mile rides)

Thanks!

New chain. I suggest SRAM. Yes to a new cassette/freewheel.

Terry makes very nice saddles.

Sounds like you have an upgraded/quality drive train and a good brakes. How are your controls? (handlebars, shifters brake levers) Have they been optimized as you have the wheels and drive train? As for a saddle I prefer brooks team pro. It will look very nice on your vintage Miyata.

You could ask in classic and vintage forum. Good bunch of people that love to give style or upgrade pointers. BTW I have a 86 Miyata 610 and love it. I ride it every day. And have spent more money on it than makes good sense. But hey it's part of the fun.

I have Shimano 105 STI shifters but the original stem and handlebars.....would it be worth it for newer carbon?

Carbon anywhere other than in the frame/fork is mostly for bling. Not worth the $$ for the marginal weight savings.

Sounds like you have 8-speed 105 brake/shift levers, but you mention a new (2 years old) cassette. Is that also 8-speed? It needs to be if it's going to work with your shifters.
If you were using the original wheels, did you have a freewheel on there? And no way it matches up to the increments in your shifter.

I've got a webpage with a rundown of how I built up my Centurion Comp TA (1984 model) into a very nice new bike.
https://www.unc.edu/~cupery/pics/bike..._TA_repainted/

Of course I have an 8 speed cassette. which i tossed on a new set of mavic aksiums.

I haven't done that since I bought my first road bike which was aluminum, full campy mirage/veloce 9 speed with a ****ty 8 speed shimano wheelset. Which cost wayyy too much.

Some of us have to learn about bikes the hard way right?

Thats a cool looking Centurion. Nice job

I'm going to put a modern stem on my bike to give me more options as the current one can't be raised enough. I wanted to throw on some anatomical drops while I'm at it and found some Kestral EMS PRO Carbon handlebars for $50. I know the carbon is just for bling in the handlebars but at that price i might as well see if it helps my hands at all when i'm riding centuries. If nothing else it will make me even more paranoid about locking up my bike.

Totally agree about the new wheels. I put a pair of Open Pros with Ultegra hubs on my 1989 618GT and the difference was night and day. I recently just replaced the original rear derailleur and potentially even chain with a new SRAM and an ultegra. Combined with the new (used) ultegra cassette I got with the derailleur, the thing absolutely PURRS now.