Rear Springs

Getting ready to replace the tired old 7.25 rear with a 8.75 in my Son's 73 Dart Swinger.Rear springs lets car sag bad in the back. What is the best performance spring to replace them on a small block ,auto trans ,street car ? Where is the best place to get them from ?
Order a coupe passenger side springs from Jegs, 312-P4120863, $131.99 each and free shipping.