Which Peugeot is it? A friend recently asked me to modify his 80's Peugeot to add a granny gear. It had standard BB but the crank BCD was weird (not 130, not 110) and the rear hub was one of those heliomatic jobs. Bottom line: not easy to swap in new cluster or cranks, and didn't want to rebuild the whole drive assembly. So I told him to just get in better shape.
I have a 70's Peugeot (UO-8) that does have French threaded BB. I converted it to SS and it is currently my favorite ride for local trips (flat around here).