The springs in my son's 67 Barracuda are made up of two sets of factory heavy duty (340) worn out springs. I took a main leaf and added two of the largest sub-leaves, then two of the next-largest, and one of the third largest. Then I built the other side the same way. They are stiff and worked reasonably well autocrossing.

I have also been known to use heavy duty C-body wagon sub-leaves on an a-body main leaf. I will be doing this with my Barracuda convertible. Ride height is a lot harder to get right, but the springs work.