The outside of the bike has some bad rust areas including the exposed parts of the bottom bracket assembly, which is one reason I though it'd be a good opportunity to replace the whole thing. I was thinking/hoping that it was the same size as an old one-piece-crank and that
this adapter would give me the threading I need. But I guess I need to measure the bottom bracket shell to see if this is really an option. But you're right, apart from the rust, I don't expect much wear and tear on the actual components, so I may end up having to just get some new old parts and use the inside cups as is.
Will it be possible at least to get cotterless cranks to fit this assembly? I'm a bit behind in crank technology, having ridden old bikes with one-piece cranks up until about a year ago. As a result, I'm afraid that I may have damaged the cotter portion of my old cranks during disassembly. It seemed pretty straightforward as I was doing it, but those little cotter pins did not come out looking especially straight.