Duster shells all the same?

74dusted hit all the major points. If you're not looking to be concourse correct, ie, you just want it to look like a '70 Duster, all you need to change on the shell is the marker lights and tail panel. The '70 front clip will bolt on, and that will solve the front markers.

The rear cross member is only different because of the cut outs for the shock mount bumper, you can put an early bumper on the later cars just by either making patches for those openings, or making an additional set of bumper brackets. I put an early bumper on my '74 Duster, you can see what I did here http://www.forabodiesonly.com/mopar/showpost.php?p=1970342487&postcount=70

The tail panel is really the most labor intensive part of the swap. I just finished putting a '71 Demon tail panel on my Duster. Not horrible, I did it in a couple days. Probably could have done it faster, but I ran into a few minor issues as usually tends to happen.

Oh, the interior panels on the later cars are plastic. So, the interior panels will still appear as the later ones unless you swap the doors over.