As stiff as you can get in rear. A stock moog spring would work or look up nightmare performance on the net and let them give you a recomendation on one of their springs. The stiff rear spring will help control the frequency at which the rear of the A-body cars want to unload. A Moroso Trick Front spring will be fine up front or give Santhuff's a call and get a recommendation from them. The front will likely be the trickiest in getting the right rate the first time and you may have to do some trimming or shimming. Savistke Custom and Classic sells spring seat spacers which will help with setting front ride height. They are at
www.scandc.com