spring rates

I have noticed over the last 4 months that most darts like mine,that havent been restored,have sagging rear springs.Ive looked at a bunch of dusters,and 2 dart sports in the junk yard for a 8.25,or 8.75 rear,and none of them seem to be saggy in the rear.Are there different rear spring rates from body styles in the a body line.I know the torsion bars have different rates depending on what engine is installed.Also,are the jcwhitney rear springs any good.If they sit my car too high,i dont mind lowering blocks,i just want the stiff performance handeling of some better springs