need new floors...

Think you're going to need a full floor pan for Duster/Demon/Dart Sport. A pre-74 Valiant floor pan might work as well. The Dart 4-door, 2-door, and HT all rode on a 111 inch wheel base, whereas the sport coupes used the 108 inch chassis. Best that I can recall, the difference showed up in the floorboard of the back seat.