Wouldn't make more sense to change the chainrings on your present crank? I assume it's a Cyclotouriste. You can still get rings for it. I just bought a 52 for $12 on eBay.
In answer to your question, you can assume nothing. Do you have French pedal threads? Will a BB for a triple be too long for a double? Not all square tapers are the same. The Japanese use a different standard. If you have French bottom bracket threads, it's really going to get ugly.