They are different. Where the 1/4 and the tail panel come together. The 70-72 have a different lip there to hide the bumper gap. There is also a different fill panel. I now I am turning a 75 Duster in to a Demon and am scratching my head about how to make it work.
Chek out my thread. It is Burnts DustDemon Build in the members resto setion. I have some pics there.