Starting in 69 springs were computer calaculated. A heavily optioned car would get a stiffer spring. YM could also fit a Z28 as a heavy duty spring. The link above was in the parts catalog to help figure what replacement spring to order.
I created the awhile back to calculate the 69 z28. Choosing different options yields different springs.
http://chevynova.org/69_RPO_Z28_Springs.html
Other Camaro and 69-70 Nova calculators here:
http://chevynova.org/calculators.html