I would start by removing the shocks and checking to see if everything moves freely. 'Cuda's aren't any different in terms of suspension design from any other Mopar from the '60's other than the fact that the rear springs aren't parallel to the centerline of the car. If everything works properly without the shocks, the problem has to be with them. When using poly bushings on the LCA's it helps to drive the "T" bars all the way into the LCA sockets so that they can't move fore and aft while driving. Good luck.