I just installed the Granatelli front coil over kit with the 300lb springs. What pound springs should I be running in the back. I currently have a sprints spring in the back but do not know the poundage.
300lbs seems pretty soft for the front end.
I assume you are 1/4mile biased.
You want to run something stiff for the rear to plant the tires.
Maybe Larual Mountain Mustang drag springs out back with an air bag in the right rear spring.
300in/lb is normally the standard for a street spring. Unless your running Bilstein or Tokico adjustables, that is about as stiff as you want to go. For the rear something in the neighborhood of the Ford "C" spring 200/300 is sufficient. If you go too much above these rates with insufficient dampening (shock) upgrade, your ride quality will go away.