I'm working on my 72 mustang and I know the rear springs are tired. I'm looking into having them re-arched and installing new bushing, vs new. Do any of you have experience with re-arched springs? I'm wondering if they will sag sooner and if they tend to fatigue/break sooner. I haven't look at the cost difference...for all I know, it's a no-brainer to go with new.