Seems to me that a 1-1/2" lowering block berween the spring and axle could be acting as a lever, allowing your pinion to climb the gear under acceleration, commonly called axle wrap.
That could be occurring regardless of your pinion angle.
IOW, it may still occur after changing the pinion angle because of the lowering blocks.
just my thoughts on the subject.

Last edited by darren (4/30/2013 5:58 AM)

Pinion angles can cause a multitude of sins, but I cannot see how it would make the car go to one side or the other.  The axle wrap-up theory, due to the lever arm of the lowering block is a good theory, but I think it is more likely the blocks are a little bit "off", with one side of the axle positioned a little ahead of the other.  This can happen if the indexing bolt on the spring does not index tight into the hole in the block, and if the pin in the block does not index into the hole in the axle perch.  There are two tolerances per each side that can add up.  Try loosening the u-bolts and moving the rear slightly/  If the car launches to the right, that would make the left side of the axle farther back than the right side.  With the u-bolts loose, use a BFH and "reposition" the axle, maybe even moving both sides as required.

A bad spring bushing  can also cause this problem.

After you check the alignment of the rear look at rerplacing the rubber bushings. I think mine are toast too after 40+ years. Mine does it too. Could me "massive horse power" going to the rear end!! How about tire pressure?
6sal6