No clue what changed but since you have a 76 engine put that fuel pump on it. Do you still have the fuel line routed over the head? That is another issue people have noted a lot is that when warmed up there is lots of problems with heat issues from the stock lines running over the head.

Reading your other thread I would also say the float in your carb may be causing some of the trouble...check and make sure it hasnt corroded through. Is it one of the single barrel holleys or the two barrel carbs? I have had a lot of trouble with the single barrel holleys on sixes and usually converted to the 2 barrel bbd carbs.