IF yours are not pitted or broken then you can have them re-arched for $100-150.
A customer of mine has had 2 sets re-arched one for a Z and one for a L65 4 spd conv both sets sit perfectly.
Here is a side by side of comparison of original Z28 springs vs Bob Harris's (Camaro Specialties) springs, they are extremely close,
The leaf ends are not quite as thin on the ends, the rectangle pockets are not quite as deep and the spring clamps are not the same gauge (thinner) - these are worlds above anything else available the other reproductions are not ANYWHERE CLOSE!
