Remeber, the spring seat HEIGHT is free...if you use adjustable collars. It doesn't allow modifications to the stock part. On my car the rears are 5", and the adjustable collar is merely loose, but held captive when the car is loaded. If it's a concern that the car will become unloaded, and the spring/collar assembely will shift, it is easy to limit suspension travel. Some guys use the shocks for that purpose, as shocks are essentially free.