Just a tip that I thought I'd pass along...something I learned from experimenting with existing garage parts.
My old 68' had some CPP rear 5" coils in it at first, along with adjustable track bar and relocated shocks.....with the front setup (dropped spindles and coils)and 18/20 wheel combo it had a tad too much rake for my taste, but worse it rode like total crap.....even with the snubbers cut.
Upon rummaging through some shelves in the garage I realized that I had a set of CPP 3" dropped coils leftover from the rear of my 70' Chevelle (these were advertised as 3" Rear Drop for 68-72 Chevelle), and upon closer inspection also realized that they were exactly the same diameter with the same pigtail as the springs in the truck, and about the same height as the 5" truck spring removed sitting side-by-side on the floor.....difference being they were a little more losely wound than they truck springs and also I believe variable rate compared to the trucks fixed rate.
Nevertheless, I gave a whirl and they turned out to work perfect! Ended up dropping the rear of the truck exactly 1" more than the 5" truck springs, but the best part was that the ride was 100% improved....no more buckboarding in the rear and it rode fantasic!
Anyways...I thought some others might like to know this tip as well. I suspect the Chevelle springs wouldn't be ideal for hauling much, but my truck didn't carry anything more than an ice chest anyways on the weekends and a Christmas tree once a year.