If you clean it, pack it with new bearings and grease, and it spins rough- get a sealed cartridge. If it spins free and smooth- keep it as is.
My view is- why replace something that works so well? Yes, you will have to redo the bottom bracket someday. But I don't view that as a negative because it is just so far off in the future. Its not like a cup-n-cone bottom bracket needs to be fully serviced twice a season for typical riding.

Those particular cranks from the mid-1970's and before would sometimes have "chain-skating" problems even with the old, 8mm-wide conventional chains, so can be even more prone to slipping (skating) while shifting down to the smaller ring if a narrower chain is used.

Initially, I try to correct the problem by first dislodging the chain so it hangs from the bottom bracket shell, allowing the cranks to be turned freely.
Then I take a long file, stand it up on one end on the ground so it contacts the smaller ring's teeth at an angle.
I then have to position my hand/arm so the cranks can turn all the way around without hitting my wrist or whatever, and rotate the cranks so the small ring's teeth are moving across the file's surface in the cutting direction.
I end up with a bevel to the tip of the small ring's teeth, which re-locates the tips of the teeth slightly toward the big ring, reducing the effective chainring spacing.

Where the beveling turns out not to be sufficient to prevent "chain skating" after a downshift to the small ring, I will then remove the small ring and bend each tooth slightly toward the big ring side, using a small adjustable wrench. I use caution to keep the bending to about a half-millimeter, which is barely visible.

Some of the SR Apex and Stronglight cranks, particularly those with half-step gearing, can have a particularly strong tendency for the chain to "skate" or slip atop the small ring's teeth, so these are the cranksets that tend to need the most attention when using a narrower 8s chain.

I will add that some 8s chains are wider than others, and that some have more outer plate flaring to better snag the chainring's teeth and resume transmission of power. The Shimano 7-8s chains are better in these regards, but are a bit wide for use on any Suntour 7s or Ultra-6 freewheels, so best to avoid the combination of these freewheels with the older Stronglight and SR Apex cranksets!

I use the narrower 9s chains on a lot of old bikes, and even they work fine on most cranksets, with at most a bit of the filing/beveling that I described.

If you create a modified stack of spacers to allow a Shimano 7s cassette to work with a Suntour 7s shifter and derailer, these will work with either the Uniglide or hyperglide cassettes. So you would be free to upgrade the hub later if you wanted, and your cassette and spacers would still be good.

The Shimano cassette hubs, even in their lowest-level offerings, are better designed and higher quality than most people might assume.
I consider their 7s and up freehubs to be the very best, and have been racing and abusing them off road for 25 years.

It was only the 600 and lower levels of their earlier 6s freehubs (the ones without the asymmetrically "bulged" hub centers) that had a tendency to loosen where the freehub body was swaged to the hubshell snout, which made adjustment of the bearings rather uncertain without securing the quick release tension to confirm the adjustment. The later 6s freehubs with the stepped hubshell center (and no oil port circlip) were extremely robust and so are perfect for any 6s application.

Adapting Hyperglide sprockets to the earlier "Uniglide" 6 and 7s freehubs requires only a few minutes with a 1/4" round file to narrow just the one widened spline on each Hyperglide sprocket. The metal cuts easily, and because there are 9 splines, one does not have to cut precisely, just get that wide spline narrowed down enough to fit and a little extra so you don't have to waste time doing any test-fits. The work goes really, really quick with a good file, it's not like cutting tool steel or anything.
Even the riveted cassettes with mounting hub "spiders" can be cut to fit the earlier hubs, but here I have used the edge of a large flat file with the cassette secured in a vise.

The modern quick-links are great for drivetrain tinkerers, allowing one to try different brands/models of chain on their experimental drivetrain for best shifting performance. The right chain can make a world of difference.

This is something I wonder about as well. My fork has a ton of room for taller tires, but the calipers I use brushed the little flashing of extra rubber at the middle of my 23mm tires when they were new (and when I take them down a dirt road that's at all wet). Is there a listing somewhere of calipers that "sit up higher on the bolt" than others?

I'd go with a nice sealed bearing UN54 or whatever because I'm lazy (putting off a cleaning/repacking of the hubs on my nice wheels for that reason).

Modern sealed cartridge bottom brackets are inexpensive, and do not require periodic service (as conventional unsealed bottom brackets do).

But there are drawbacks that make it worth considering to retain conventional bottom brackets that are of good quality, and still in good condition. These are the major reasons that I retain conventional bottom brackets that are still in good condition.

#1 Sealed bottom brackets are not serviceable or adjustable. When new in the box they are always stiff with a lot of rotational drag. Stiffer than a conventional bottom bracket that has been properly adjusted. Sealed bottom brackets have drag from the grease that they contain, AND the adjustment it fixed so if manufacturing positions them too tight, there isn't any way to adjust the bearing play. There is no way to tell a good one from a bad one when new in the box.

#2 Sealed cartridge bottom brackets are HEAVY. Heavier than a conventional bottom bracket that sells at the same price range. When you jump to the $80 and higher range, sealed bottom brackets get lighter, but are still heavier than a $30 conventional bottom bracket. When I was on a kick to replace my conventional bottom brackets with sealed bottom brackets, the UN51 was about $25, and the UN72 was about $75. The UN72 was easy to identify with it's hollow chrome plated spindle and lighter weight. I also think that there was more attention to assembly, as the more expensive sealed bottom brackets spin lighter than the cheaper UN51, but still not as well an adjusted conventional bottom bracket.

It was exactly one of those early non-bulged 600 hubs that scared me off - the bulge, if I understand, covered threads to secure the freehub body, whereas the old type (mine) was press-fit and then secured by the cup/cone/bearing assembly of the hub itself. Good to know I can look for a nice Tricolor/105 hub with the right spacing and build up the cassette to Suntour specs. Thanks much.

I'm really glad this worked out! I was just going to chime in. I definitely can't speak to all the nuanced knowledge of what parts can work with other parts for which they were not originally designed or how to get by with buying the least amount of parts needed to get a bike functionally upgraded (such as moving a vintage bike to brifters without replacing the entire drivetrain). But I also don't have the taste for hacking together parts from separate brands and lines. Call it OCD or laziness. I'm not sure which. I tend to try to do all my builds with a single groupset (same brand, same or close to same line). It was all designed to work together and provided you are selecting the right group, it works famously.

And my input on this matter is just as you experienced. If you have the time, with patience, a solid, modern groupset with brifters can be had at very reasonable prices. Even on eBay. My recent builds used Tiagra 9-speed, Ultegra 9-speed, and Shimano 105. I got each of these for $150, $230, and $99 respectively. Each came with everything (fd, rd, crankset, bb, chain, cassette, brakes) and a couple even had the cable stops and cables and housing (hardly used) with them, and other bits. I prefer this approach partly because the end product comes out very cohesive visually, and partly because the compatibility is a given.

So, while I know things can be done and for much less money, for me, the end result and ease of installation (sans-hacks) is worth the extra money. I have also found people respond stronger to builds that maintain this cohesive drivetrain (it looks less franken-bike maybe?) so resale is a bit better too.

That's the hope! I'll use it as a learning lesson to clean, lube and overhaul the components the components that need it. The brifters are in working order, but I am not sure how smooth they are yet. The interwebz tells me it's best to hose out the old grease and re-lubricate, so I'll probably do that.

I also found some surface rust on one of rear drops that is large enough to warrant blasting/sanding and paint. It doesn't show it in the photos, but the current paint was rattle-canned on over the factory paint job with uneven primer. It's already chipped down to the metal in quite a few places, so I think I'll look around for some powder-coating shops. Here in SD there are many deals to be had in that department. RAL-5015 looks like a sharp color for the bike, and I'll ditch the yellow handlebar tape for a darker shade; after tonight's loss, the Nishiki told that me it wants nothing to do with the Chargers.

Waiting for parts. "Build" section of thread will commence after then. Should be fun!

Finally picked up my parts! As expected, the parts looked used, but in good working order.

Calipers (BR-A410)
- Good shape, no scratches. Clean, other than some surface crud. Pads are hardened, but I already have a new set ready to replace them.
- Both calipers came with their anchor bolts.

Bottom Bracket
- Doesn't move too smoothly. Also missing the outer cups. I'll eat the $15-20 and buy a new one, or maybe clean and re-lubricate my current unsealed bracket if I can fit the crank arms on it. It moves quite smoothly as-is.

Crank/chainrings (A410 cranks with 36-46t Hyperglide)
- Good shape. No bad wear on the ring teeth that I can see. No noticeable warp in the rings. Some surface scratches on the crank arms.

FD (FD-A410)
- Dirty, but otherwise looks good. The outer cog does not move nearly as smoothly as the inner cog. No scratches at all on the painted parts. After a clean/polish and adjustment, it should look and ride like new.

RD (RD-A410)
- Again, dirty but fine. No dents or bent arms. One scratch in the paint.

STI Shifters
Good:
- The left clicks smoothly.
- The right sticks before clicking, but moves up/down through all settings. I'm going to follow a couple of the guides I've found to flush out the old grease and re-lubricate the internals.

Bad:
- The hoods are in rough shape. The right hood even has a tear in it on the top. Any tips on getting replacement hoods?
- There are more than a few scratches on the front of the shifters. I might be able to wet sand and polish the faces and remove the scratches. There are also a few nicks in the lever paint.

--

While I hunt for a powder-coater (San Diego recommendations appreciated), I'm going to turn to cleaning the Shimano parts before they go on the Nishiki.

Started tonight by removing the chain rings from the cranks and cleaned/degreased the rings and crank arms. There are some surface scratches on the cranks that I think I'll sand polish off, given that I will be working on the shifter caps anyway.

I'm pretty happy. I know it's not the shiniest gear out there, but it'll do me just fine for my purposes and I'll enjoy cleaning/servicing the parts and learning more about my bike.