Just to make sure I didn't get an oddball crossover, I checked the bottom bracket shell:
Seems like 69 or 68mm instead of 70, which had me concerned. Probably just faced-down that much after assembly.
12 threads in 1/2 inch, check: 24TPI. And right-hand threaded on both sides, that impression was from putting the paper through the shell.
I'm confident now that an Italian BB will work, and modern Shimano's are reasonably priced. I think that's my plan. I'll wait to order until the drivetrain is more settled out, since those aren't going anywhere. That will give me the best chance of getting the correct spindle-length on the first try.