I have the original hard cover manual for the crossbow plan on this site. (family heirloom,Ah!)
I'm hoping to contact someone who actually built the steel crossbow and used a car spring in the 7/32 to 1/4in thick range. Most cars today have either coil rears or 3/8 springs.
However, i did find a set of sno mobile runner springs of 24inches X 5/32 thick, which come with a doubler spring of 12inches of the same thickness which acts has a shock absorber. The clincher with this setup is it comes with two holes spaced 3in apart instead of the one center.