This may sound funny but think about it before you laugh. Are you sure your car is sagging to one side or is the other side too high? Yes I know it sounds ridiculas but everyone that sees my car tells me it sits almost 2 inches high all the way around. Get a judging manual and see how high it really should be, then you will know which way to go. I have been told by a restoration shop that I have no shims in the rear of the car ..... so now where do I go? Can the newer neopreme spacers be that much larger than the correct spacers that they raise the body that much? I don't even know yet if I have the "correct" spacers or something different.
Anyhow, make sure if your car is tooo high or is it tooo low.

I think I have the answer to The leaning 1974 of pisa...A friend of mine also has an 1980 Corvette with this leaning problem and he had his entire rear apart an concluded that his leaning problem was in fact the rear trailing arm shims (that adjust the toe in) were rusted together causing this lean....