I am the master of weird problems.

I bought a nice pair of Super Stock springs from the swap meet. I was told they are for an a body. They bolted on easily enough. However, when I let the car settle, the rear shackles pivot backwards until they butt up against the frame crossmember. This also causes the arch in the springs to flatten out lowering the car back to stock ride height. Any idea why this would be? What can I do to fix this? Are the springs the wrong length?