My 64 has that problem also. It improved some when I replaced the rear shocks and springs. I still need to replace the front same and maybe then it will sit level again. Those are about the only 2 things that I know would cause that.

You can check Leaf springs and coil springs and see if they have them for your car. They were the cheapest I found for leaf springs on mine so might be the same for yours.

Deb