From my experience rebuilding my 68 GTO rear end, should the springs sit inside perches with an insolator on top a specific way? Maybe you dont have the springs seated quite right? I know when I did my GTO everything would fit and line up just fine but if I rotated the springs and line up with the perch notch it fit even better.
Just a suggestion as I know the "alignment notch" on the GTO was about 1/2 inch which would be enough to throw play into the back under load and accelleration like you describe. Mind you I havent seen the Camaro so I am speculating.