[Jack21]

These spring rates sound about right for logging trucks, but for a Cobra?

Rear springs 180 - 220 #/" (street), 220 - 240 #/" (race).

Fronts are a little stiffer because the control arm acts as a lever on the spring. Load on wheel ~ 600#, load on spring ~ 900#.
350#/" for a small block, 500 #/" for a big block.