Well first you need to consider your axle placement. need to measure the main leafs on all 3 and see where it is going to locate your axle. once you find one that puts your axle where you want it with a good shackle angle then just add the leafs to it for height and comfort. Typically between 3 -4 leafs per side. set the truck down on it and see how it sits. keep in mind that the springs will settle almost an inch or two after about a week or so. Hope that helped a little.
