I am trying to replace the rear coil springs on my 1968 Buick GS 350 California. the old springs have a free heighth of about 16.5, and all of the spring I look up for it have a free heighth of 12.5-12.9. I tried intsalling them, but they drop my rear end too much. I don't know if the rear end has been modified or not. It does not appear to have been. I'm not sure what to do. Please help! Any advice would be greatly appreciated.