I wouldn't use those clamps, nor blocks or anything else to change how a spring, especially coiled determines the ride height. Remember, we're dealing with a 41 year old car here that probably hasn't seen much road time as of late, and the ball joints are not going to react vaery nicely to a change in alignment angles and a couple of extra hundred pounds per coil when that clamp is on there.
The correct and safest way to go here is to determine where to find a leaf spring for the rear with less stacks (sometimes you can remove form the stocker), and a shorter spring for the front. I don't even like when guys say they are going to cut it. That creates the potential for damage and possible injury when the spring or related suspension fails more than removal of the spring itself, which isn't bad, no special tools required ,other than a good hunk of chain to keep that bad boy at bay.
