Help! What car is closer in shape to a 1973-76 Dart Sport The 1972 Duster or 1972 Demon?

3 inches should not matter on a car cover.

If the one you have is for a "23" body, like a regular Dart, Valiant, Scamp, etc, then it may not fit the rear window area properly.

A "29" body Demon, Dart Sport, or any Duster are virtually identical in shape, so will interchange on a car cover for sure.