Start looking at the width of the frame at the point where the upper links attach, and write this measurement down. Subsequently measure the distance along the axletube from just shy of one brake mounting plate to the other. This will be the distance apart of your lower-link axle-mounting points. If at this point you are starting to really like the idea of leaf springs, then we congratulate you for having some common sense. If you are still thinking that you'll be the talk of the town with your new super four-link suspension, then dig out that piece of graph paper and sharpen your pencil, because there is more work to do.
If you have a pile of graph paper crumpled into balls and a headache from thinking too hard, then you are right on track. The tricky part comes when you take your drawing and see if you can actually attach the links you drew on the truck's frame and axles. This is where compromise comes in. You may need to move the fuel tank, exhaust, or various other low-hanging parts of your truck. There is always a bit of adjustment available. The upper links can be slightly longer or shorter than 0.7 (70 percent) of the lower links, but try not to pass 0.6 (60 percent) or 0.8 (80 percent).
Figure 4
On the graph paper draw lines up to the top of the paper from the front- and rear-axle centerline, upper- and lower-link axle, and frame-mounting points. Next, in the upper space draw the rear axle from the top view with the lower-link axle mounts plotted at the distance apart you measured from the actual rear axle. Follow that by drawing in the framerails from the top view with the center of the frame over the center of the rear axle. But wait, there's more.
Figure 5
Draw in the upper links first. Remember to have them start from the frame, but at the axle keep them slightly separate to allow access to the nuts that will go on the ends of the bolts running through your rod ends. Now grab an angle finder. It is very important that the angle of the two upper links be no less than 40 degrees. This angle is what locates the axle laterally or side to side. The smaller or more shallow the angle, the weaker the lateral control. This again may mean shortening the upper links, but try not to make them less than 70 percent of the horizontal length of the lower links. If need be, you may need to shorten the lower link's horizontal length as well, but try to keep them as long and as level as possible. If you are wondering when you get to start installing your really cool new coilover shocks, then you might need a lesson in patience. Get another cup of java and keep studying.
Figure 6
To reduce the rear steer of the axle, we need the tire to move towards the center of the frame, side to side, as it articulates and not towards the center of the frame, front to back. First you need to draw in lines extending from the links until they converge when viewed from the top of the vehicle. These convergence points are known as the lateral constraint points (LCP). The upper-link's extended lines will most likely converge at a LCP just behind the rear axle. The lower-link's extended lines should converge at a LCP somewhere forward of the transmission, depending on how much of an angle you give them when looking at the top view. An angled set of lower links helps the upper links locate the axle laterally and fight rear steer, but also requires a larger area to slide over obstacles. The best route seems to be a slight angle, but not at severe as the upper links. An acceptable angle will have the lower link's separation at the frame equal to 50 to 70 percent of axle-mount separation, which may require fabricating a crossmember to mount them to, as discussed earlier.