I'm looking at CJPonyParts website and they have some lowering blocks by Scott Drake for '67-'73 Mustangs. They come with longer U-bolts as well.
Okay, 'finally got the lowering blocks installed (1-1/2 inch) and the car sits proper now, however because the axle housing was raised up, the Addco rear sway-bar now won't clear the right-rear shock. I just took the damn thing off. It never fit well anyway. Stupid aftermarket crap. BTW: the centering pins on the aluminum lowering blocks (from Speedway) were 9/16" in diameter, which were slightly too big to fit in the holes on the axle pad. I didn't want to drill out the holes, so I had a friend machine the centering pins down to 1/2", which is the size of stock centering pins. Perfect fit.