The trunk lid will fit fine. Same exact piece as on a Duster. The only differences in the trunk lids are the reinforcing rib on the later ones and whether or not it had cut outs for the wing. Unless you find one from a Dart Sport "Lite" or Feather Duster, in which case the inner bracing is aluminum.
Doors are also the same as far as fit and function goes for the Duster, Demon, and Dart Sport. The later 73-76 doors have different inner handle locations, plastic tops instead of metal, and different interior panels. But from the outside the door is the same. Window regulators and guts are also different depending on the year, but again, from the outside they're the same. So, if you swap 73-76 doors onto a 70-72 car, your interior panels won't match. The windows, hardware and regulators also changed depending on the year, so its best to keep the window and guts matched to the door.
In fact, the doors on all of the 67-76 hardtop Darts, Dusters, Scamps, Valiants, Demon's, Dart Sports etc are the same from the outside. The only one that doesn't interchange is the Barracuda. But, all of the window guts are different going from a Dart to a Duster, even the later 73-76 Darts without the wing vents. The regulators and tracks don't even mount in the same place, so if you swap a Dart door shell onto a Duster you'll have to drill the mounting holes for the tracks and regulators. But believe it or not, the door shell itself is the same. Same issue with the window hardware too, much easier to keep the windows and hardware matched to the year of the door.
Rear bumpers interchange between the Dusters/Demons/Dart Sports, but only if you keep the year range right. 70-73 will interchange, and 74-76 will interchange. 74-76 had shock mount bumpers, so you can't bolt them onto the earlier cars without some work. Putting '70-73 bumpers on 74-76 cars can be done, but you need the earlier brackets, and you need to drill some holes and make a bracket/weld in a patch. I converted my '74 Duster to a Demon, and swapped on the earlier non-shock mount rear bumper.
On that note, the front sheet metal will interchange, but only as a whole clip, you can't mix and match . You can swap everything from the cowl forward, which changes whether or not your have a Dart, Duster, Demon, Dart Sport, Scamp, Valiant, etc, depending on what year and model car you start with and swap parts from. I turned my '74 Duster into a '71 Demon tribute, you can see everything I did in my build thread. There's a link in my signature.
