It sounds like you have a "civilian" 80's Caprice.
The biggest bang for your buck is to transplant in the suspension from a 9C1 (Police package) car:
Rear Sway bar (and front if its larger, I don't know for sure). New rear lower control arms (from the '91+ Service Bulletin kit) will provide a good method of fastening the rear sway bar on at an affordable price (works fine with the factory sway bar; not so good with any aftermarket rear bars).
Ideally, 9C1 front and rear springs.
Bilstein 1104 & 0929 shocks (which were an option on the 80's 9C1 cars).
I wouldn't bother with Poly bushings for the front swaybar; they squeak if not lubed often, and for your application will not be worth the bother. Stick with factory sway bar bushings.
You will be amazed at the improvement, for a modest investment.
My daily driver is a 1990 9C1, set up exactly as above. Its a joy to drive, with a much better (firm) ride than my stock suspension '96 SS. The SS is a wallowing pig in comparison.