I've run into an interesting problem. On my 68 Firebird the leaf springs were replaced at one point in it's life from the factory (I think) 5 leaves, to 6 leaves. This didn't leave enough room inside the perch and shock plate. They torqued the hell out of the bolts until the plates were bent and the rubber squeezed to nothing. There was still a considerable gap between the plate and perch.
I removed one leaf, about 12" long, from the bottom, and added a 1/4" shim between the perch and plate. Now, the new rubber has a nice clamping pressure without too much distortion.
I also sandblasted the springs and painted them. I didn't take the leaves apart. You can see the before and after pics below.
Now the car sits way too high. They are almost at the limits of the shock travel. The shackle and front mount bolts are still loose.
That's all I did. I can't think of any reason why the springs are not compressing like they used to. Anybody have some suggestions as to why this is happening?