I currently use rear springs out of a 70-72 Monte Carlo.They are a bolt in,but they are soft.I use air bags that you can get from Jeg's for $78,Part#022-60844.They fit inside the spring and keep the coils from compressing too much.They are adjustable from 5psi to 35psi.You can have a nice comfortable ride,or you can air them up to 35psi and put 3000lbs in the rear.Very adjustable,and they work real well for drag racing.Just my .02 worth.