First thing to do is to check the actual rear suspension height to see if the spring is still within specs; measure from the center of the inboard and outboard camber strut rod bushing bolts to ground and calculate the difference between the two, and see how that compares to the "D" height shown in the chart in UPC 4 of your Assembly Manual - should be around 3-3/8". That measures actual spring deflection under load, independent of tire size or other variables. Two-minute job with a yo-yo. If it isn't more than an inch off, usually just disassembling the spring, cleaning it up, painting it, installing new liners (Quanta), and re-assembling it will bring it back pretty close to specs.
If it's more than an inch off (or looks cambered "up" instead of being almost flat to cambered slightly down), and isn't badly corroded, Eaton Detroit Spring (
www.eatonsprings.com ) can re-arch it correctly, with proper heat-treat, annealing, and shot-peening, just like new; tell them not to paint it - paint it yourself and assemble it with new liners.
Most of the reproduction springs the vendors sell have too much arch in them and tend to make the back of the car sit 1"-2" too high.
The pics below show how a correctly-arched rear spring should look, installed and under normal load; if your spring is cambered "up" instead of "down", it needs re-arching or replacement.