I run between 55 and 90 lb/inch spring rates at the backs of my altereds. The softer the spring rate the less energy is released when the springs move, which I think is desireable in a light car. On one car I actually took a set of helper springs off a LoadLeveler shock (55) and used them. With your shop capabilities I am wondering why you don't narrow your own housing? I've done a few as shown.