I had a '74 Duster that had one side 1/4" closer to the front of the opening than the other side. Original panels, etc. I checked the wheelbase and it was equal on both sides; the body was just welded together that way.
First thing you should do is check wheelbase, and take a triangulated measurement to make sure the rearend is square in the car. If it's been a racecar, the rearend housing could also be bent. Make sure the springs have the same ft section length, and that the rearend is centered on the pins in the springs.