I had a 69 Formula S Bararracuda that got slammed and I had the bright idea of swapping those springs to a 67 Fastback[not Formula S]. I thought I would get the same ride height as the 69 car was lower and handled ten times better. I found out the rear frame where the shackle connected was set back about 3 inches on the Formula S.That spring had a preload that made the spring have to be flattened out in order to reach the shackle. The 67 car sat at least 3 inches higher with a significant arch in the spring. The 69 sat lower with almost no arch. If anyone knows of a formula car and has a regular A-Body I'get out the tape and I think you will find the attaching point on the formula is farther back, flattening the spring for better handling.