Lowering blocks are not recommended because it increases the rotational torque applied to the leaf spring ending up with increased axle windup. The thicker the spacer the more the issue becomes. No free lunch.
Hey John for years I have used lowering blocks on most of my classics the blocks seem to be about the same. I do only purchase the kits with the nylock nuts for the ubolts. I do burnouts and make passes on the street but no track time. Yes there is some wheel hop but all these years I never had a problem just weekend daily driver fun. All the blocks are generally going to fit the same top of block in the centering hole and the bottom of block on the leaf spring pin.