You're side loading 40+ year old springs by a 1/2" just to the perches. The results are going to be a crap shoot. I don't understand why people insist on installing the B-body rear axle without moving the perches. You're losing tire clearance to the springs and inducing all kinds of binding in the hangers and shackles. Installing new bushings and leaf springs could just as easily make your problem worse, you'll get more binding because there will be less play.
Moving the perches is easy.