Putting in an 8 3/4 rear in my duster that has a 7 1/4 rear parking brake cable question.

Nope, same cable.....9", 10", 7 1/4, 8 3/4 , Dart, Valiant, Barracuda.... It's usually the front cable that is different depending on the wheel base, except for the '66 and earlier A-bodies where they change the intermediate cable. There's plenty of adjustment to account for the difference in brakes size.