The U of Nebraska Ethanol truck had a 5.3 that was upped to 5.7, worked...still works fine, one of my friends bought it when the university auctioned it. That motor was done in 98!
A 76 can feed 900+ dude. If you can find a 5.3 for pennies, do that. Get it running, then do cam/tune to go along with the headers and other small stuff that you'll do on the install...