We did the same swap as you on my sons 75 valiant to make it look as a 72. The entire rear section with the bumper support braces have to been used from the 70-72 model. Here are a few pics. This one is of the finished car. http://i222.photobucket.com/albums/dd145/ray79rt/72swinger.jpg
These are of the car cut up during the swap. We did not drill the spot welds because the donor car had rust in that area so we cut an inch or so from the edge and butt welded. The extensions hide almost all of the cuts except for were we cut it across the trunk opening at the top. We also replaced the truck floor at the same time. It is not a bolt on job but we did a good job and I am not a pro body guy. http://i222.photobucket.com/albums/dd145/ray79rt/100_0714.jpg