needing a new rear end and advice

Valiants and derivatives (Duster, Demon, Dart Sport) were 108" wheelbase. Darts and Scamps were 111" wheelbase. Whichever rear end you go with, all you have to do is take your current drive shaft to a competent drive line shop and have it shortened to suit. All they will need to know is which rear you are replacing and which rear you are replacing it with. If you haven't replaced the front and rear U-joints yet I would have the shop do that as well. If you have replaced the U-joints, no worries, your rear joint will bolt up to the new rear end. The U-joints didn't get bigger until big blocks came out to play...