How much sag do you have? Measure it, and figure from there. There is a wide difference in what amount of sag will happen due to spring rate and shackle angle, plus shackle length. You should end up with at least 1/2 the shock travel left when the car is at rest, and preferably 2 /3. Some cars rise a little, some setups will really lift the back. I have had both extremes. You might want to consider some adjustable shocks like ranchos. Best though to start a thread listing your current and future combo to get an idea of what to do. On my Dart I have been through five different sets of shocks, each one better as the power went up. Hopefully you can spend less than I did!