As I understand it (someone correct me if I'm wrong), as springs get old and de-arch you loose caster. It also puts your drag link out of level. Now your truck is going to handle bad, and terrible on bumps. Now, you could put more shims in to correct the caster angle, but that doesn't fix the drag link problem. The right fix would be to start with new front springs. I will admit that I'm running 4 degree shims to correct the 1 degree of caster I started with.
I may be wrong about the whole 'de-arching' thing, but that's how it was explained to me and that would explain why so many of these old trucks don't have the caster angle they should.
