I think it comes down to trial and error, or testing, testing, testing!!
There are lots of factors involved, automatic or stick?? transbrake?? weight of vehicle?? desired goals??
I have a 68 Torino, same basic rear suspension setup as your car.
My car is HEAVY!! I have heavy duty leaf springs that consist of five leafs instead of the stock four leafs. They seem like "hard" springs to me.
I have Cal-Tracs bars on it, and it is an automatic without a transbrake...I just "foot it" 8).
As the world turns...I get more dizzy!