Interesting. Usually guys change them to get a little lower stance. They are also a little stiffer than the small block springs, for better handling. Both of those cars have larger tires on them. Mine are 225/50/16 front, and 245/50/16 rears. I had 235/60/15
all around and the front ones rubbed on tight turns into a driveway, not on the road though. They were orginal equipment on the Z/28s.
Check out David Pozzi thread on springs. He has a comparative chart that is really amazing.
When you change them out make sure you follow the AIM instructions and get them situated correctly, and the car will set nice and even on all four corners. I had a spring shop compress them and band them. My installer insisted on it. He was doing a disc brake conversion for me at the time. It made for a perfect installation. 20 bucks for the banding.
Oh and another thing. I could not find Moog 6308s, but Napa had a cross reference to a Dana spring, exact specs. Napa 277-1984.
This is with the 15s. PS I have 4 leafs on the rear.
Side shot with the Vintage Wheel Works and 50 series. Nice tight handling and no rubs.