Are Demon and Dart seats different?

The Dart cushions are taller (thicker) and longer (front to back) because the Dart has more headroom (due to a higher roof line) and more legroom
(3 inches longer wheelbase) in the back.

Courtesy of the Hamtramck Registry.....
Check out the right bottom corner of the page.

http://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1971/71_Dart_Demon0026.jpg