I am curious as well, I purchased the SS springs that were mentioned earlier in this thread. I'm building and old school handling car and everyone says go with the SS leaf springs, as big of torsion bars you can get, redo the bushings, and add at least a front sway bar.
I wonder if the springs on your 65 were replaced at different times? Is it possible one side broke and was replaced and the other wasn't so now it's weaker from more stress cycles?