As you know, I had the Pinto based kit that I was going to build with the Buick V6/TH200R4/8″ Ford rear drive train. I ended up assembling the rolling chassis, shrink wrapping the body and the parts on it, and selling it to a Sacramento, CA buyer.
You may also remember I was missing the rear spring plate mounts and had to fabricate my own. I also bought new quarter-elliptical leafs from Eaton-Detroit Spring (http://www.eatonsprings.com). I bought the Pinto application semi-ellipticals and had them cut them off to quarter-elliptical. Why pay for shipping the extra weight. Also, they can cut them a lot easier than I could. You’ll need to know the front spring eye to springs center bolt measurement for them.
I also did not use the lame CMC upper link that keeps the rear from rolling. I fabricated two adjustable upper links (control arms) that were mounted directly over the springs. I used swedged tubing, heim joints and some chassis tabs to weld on the rear. (http://www.speedwaymotors.com). This allowed me to adjust the pinion angle perfectly.
If you want, I’ll send the pics I took during the build. I actually had to remove 2 of the 4 leafs from the new springs too. Otherwise it sat too high and was too stiff.
I think you have a dead 30 year old spring problem. Another “fix” could be air shocks or coil helper style shocks. Also, the spring plate mounts could be modified to make the “shackle” part longer, thereby raising the body. You could even put multiple holes in the longer “shackles” for adjusable ride height. Your frame rails should remain very close to parallel to the ground when you and your passenger in the car.
If I was keeping the car I sold and continued the build with the Buick drive train, I was going to fabricate a coil-over or air bag 4 link set up with a panhard bar.