Have you considered a rear frame rail brace to tie the back end together?
Scot W at GNS Performance has these brackets available. Due to the stellar quality of factory tolerances in the day, you'll need to bolt these on and measure up and weld in some square bar stock to suit your frames unique dimensions. Powder coat the finished product and you're good to go.
I've got this installed on my G Body and feel it helps a lot.
http://www.gnsperformance.com/bodyframe-g-body
G-Body Rear Frame Brace Brackets
The new rear frame brace kit is designed to stiffen the rear frame ends
considerably over the stock frame. This bracket kit allows you to bolt each bracket onto each rear frame end then you are required to make your own cross bar to fit your specific length (as each frame will have different measurements). Once your cross bar is measured and cut to length it can be "Tack Welded" in place then removed and finished and coated how you like. Fits 78-88 G-body
$95.95
