There’s nothing to “buy”, with radial tires and the physical world we inhabit these cars are substantially undersprung with factory torsion bars. There’s no equation or modern method of determining wheel rates for an application that will tell you your car is correctly sprung with the factory bars and a modern set of tires.
Your car, run it how you want. But your car IS undersprung, and it no doubt handles like it is.
@Mattax is right, the A’s didn’t come standard with radials. The complete changeover for Mopar was when the F’s replaced the A’s, the F’s got radials standard. And, surprise, they also got a redesigned suspension to go with them to correct the differences between bias ply and radial tire use.