Ok, I ended up doing a clear nitro refinish on the body. Here are some pics of the process. The only thing I'm slightly unhappy about is that the difference in grain between the three pieces of the body is much more apparent without the degree of opacity that the original finish had. If I would have known that I might have opted for a solid color instead. There are also some areas that weren't exposed (for many years I imagine) - under the pickguard and where the bridge ashtray was left on. The wood in those areas is a lighter color, and sanding didn't even it out. Not a big deal where the pickguard went back on, but you can see it around the bridge. Pro finishers probably have a way of dealing with that but I just left it as is.
I used a heat gun and scraper to remove the poly, and as you can see it came off pretty easily in sheets. It was interesting that nearly all of the dings and scratches were only in the poly and not in the wood, which is a testament to the durability of the poly. After the poly was removed there wasn't much sanding needed to get it ready to finish.
I started with a couple of applications of Stew Mac grain filler and then about 4 coats of Stew Mac sanding sealer. Next were a few coats of amber tinted nitro and finally about 6 coats of clear gloss nitro from Ohio Valley Nitro. I sanded in between each step with 600 grit and 400 for some of the tougher spots. After the clear coats I wet sanded with 1000, 1200 and 2000 and then did progressive levels of polishing compound to buff it out. There wasn't much of the latter needed because there was already a lot of gloss after the wet sanding.
There might be some confirmation bias involved, but I think the bass does sound more open with the thinner finish. The G string in particular had a kind of muted quality before, and I don't hear that now. After seeing the thickness of the poly when I removed it I can understand how it might act as a tone-choking device. After the body was stripped, knocking on it produced a noticeably more resonant sound than before.
The bass is stock except for the following mods: Hipshot D-tuner, vintage style threaded saddle bridge (I don't like how the saddles on the '70's bridges move around so much from side to side), Dunlop Straplocks and a .047mF orange drop cap on the tone control. I find it much easier to play without the tugbar and the ashtray pickup and bridge covers, so those are removed and stashed away.
I took the bass on a rehearsal and gig late last week and it sounded great! It is definitely one of the best late '70's JB's I've ever played and well worth all the effort to get it looking good.