Dying 7.25 rear end swapping to a better rear end.

OK , you say you converted the front from drum to discs . You didn't say the front is still small bolt pattern or now large bolt pattern.
I'm going to assume you now have late model large bolt pattern front disc brakes.
Large bolt pattern 7.25 rears from much later motel a-bodies are a dime a dozen. They bolt right in to your early A with very little effort or expense.