this is true... the rx-7 rear ends are pretty decent though, if you get the turbo or GTUs versions that is... but it sounds like you know your stuff so you should be able to make a lot of stuff work... I don't know a ton about the drive shaft, I would think you could just put a 7 drive shaft in but maybe fcdriver knows more about it than me haha good luck though, and don't put it in anything front wheel drive or heavy hahaha
