Fenders, hood, headlight brackets, and bumpers are different. Grille and headlight trim is different, too. The electrical connections to the parking lamp/turn signal are different. Also the 72 model had a valence panel under the bumper. If you get a full 72 front clip, you should have everything that you need. As I recall, the light harness is part of the engine harness. If you swap that out, you'll have no problem with the parking lamp/turn signal connection.
For what it's worth, I plan on a similar swap in the future with my 73 Dart Custom 4-door. The 72 grilles with ALL of the attachment points intact are very hard to find. The grilles and headlight surrounds are unique to the 72 model. They are essentially the same for the 70 & 71 models.
