How fast can a stock leaf rearend be?

My little '65 Dart has 275/60 real street tires and the HD springs from Mancini. It is a mild 360 LA, 904, 4.10 gear. After going to test and tune last night, it seems that the springs are actually too stiff. The thing spins all the way through 1st and into 2nd. I am actually thinking about going back to stock with the front half clamped and aluminum front eyes. About 40 years ago, I ran a similar setup on a 1957 Dodge (big car) and it planted the tires so well that the rear of the car actually raised on launch. Any feedback on this idea?