Not to hijack this thread but we're in the process of trying lowering shackles on our '75 and here are the issues we found with the O'Reilly sourced (Superior Performance 13-4100)pieces - applies to the earlier trucks like ours:

Original shackle bushing length measures about 2.85" vs 3.02" on the replacement - so the hanger would have to be spread about 1/8" for it to fit in there. Not a big deal - it was probably crushed that much when the factory built it. But the deal breaker is bushing ID. Leaf spring bushing ID is 1/2", while the new shackles measure 9/16" on all holes. Could enlarge the original holes are replace the bolts but I cannot find a proper size bushing for the leaf.