Help *** Alternator ***

[406chevy]

I have a 71 chevy truck with new 396. My original alternaotr went out days before i made the swap to the 396. So i replaced it with a stock one from the Oriely Auto parts. The alternator was working when installed( discounnected the battery while running and see if it keeps going). After two days it was no longer charging. Went back to store and got another one, this time highperformance model. Same thing happens after 3 days of driving. it is no longer working. Had them check at the parts store, they confirmed that both were not charging when i returned. What is knocking them out or what should i do to fix the problem for good.