I have a square tapered bottom bracket which I am trying to remove but the fixed cup is proving difficult to loosen. The bike is a Peugeout Princeton from the early 90s. I have heard older French bikes are not reverse threaded, would this likely be the case for this bike?
A large vice can be used to clamp the cup and the frame should provide enough leverage, especially since you've almost completely disassembled the bike. Use some penetrating oil from the inside of the BB. Give it some time to seep into the threads. A hair-dryer could be helpful but a hot air gun might damage the paint.
French BBs are usually right/right and on assembly, thread-locker may have been applied to the fixed cup. If the right cup is 35mm, the BB is French. The British standard is either 1.370" or 1.375", according to Sheldon Brown. https://www.sheldonbrown.com/gloss_bo-z.html#bottom
The article also gives suggestion for removal of the fixed cup.