I thought I'd circle back to this post and tell you what I decided to do. I measured the drop-out with a caliper and it comes to 127.8 mm. Took the bike to a local shop and they let me put in a 130 mm wheel. I honestly didn't like the spreading of the frame - it was fiddly to get it in there and I just didn't like the idea of the stress on the frame. So I asked them to re-space the wheel to 128 mm and re-dish. After struggling on 17 miles of relentless uphill last weekend in Olympic National Park, I decided to trade in my 6-speed 24T freewheel for an 8-speed 32T cassette. But that required a new derailleur and new, longer chain. So it's been an expensive adventure so far, but I expect it will ride easier, brake much better, and be more serviceable. I really like the idea of switching to brifters sometime in the future and a new front wheel would be nice too, but for now, it will be friction downtube shifting for me. The service guy asked me if my riding needs had changed. I told him, "I'm not 28 like when I bought this bike!" (I'll be 60 this year.)
Thanks for the comments, I learned something new every time I re-read this thread, after more research that let me understand it better.