If it's the G-body in the picture that you're talking about, have you inspected the frame for rust? These cars are notorious for rust in the rear frame rails. Second, inspect for cracks in the spring pockets in the frame that could be allowing one side to sag. I had this happen on a 70 Supreme. Third, inspect the rubber body mounts as well as the tabs for them on the frame. Again, deteriorated rubber or rusted mounts can cause a sag. Finally, have you verified the problem is with the frame, or the body? With the tires properly inflated, park on a flat, level surface. Carefully measure from the ground to the underside of the frame rails at a number of reference points. Compare the two sides. If the frame is parallel to the ground, the problem is the body.
Oh, and you DID remove the 455 block from the trunk, didn't you...
