7 1/4 rear end hold up?

My bro in law put a camed up stock 69 340 w ported U heads in a 72 swinger w a 7 1/4 and 904. The rear spider gears broke 2 weeks into it and then the 904 blew a few weeks after installing the 8 3/4. Might as well do it all @ once or you will be chasing the weakest link. My bro in law was driving a little hard. If you have the power you will use it eventually