How to shorten the wheelbase on a big truck. Determine the wheelbase in its present configuration, and the wheelbase you need. A minus B = C.
Drop your driveshaft, and have it sent out to be shortened by measurement C. Do this now, as it can take a couple days for it to be done. Take the u-joints out of it to save yourself some money. You will be replacing them with new ones.
Next, support the frame rails at their present height, and measure that height. Record that measurement also. Measure center to center, both axles, both sides. Measure from the center of your steering axle to the center of your front drive axle, and then your steering to the back axle. Record these measurements also.
Drain the air tanks, and disconnect the battery.
If the crossmembers and spring hangers use huckbolts, you will need to remove them with a cutting torch, or zipcut blades on a grinder. The torch method seems like the obvious choice except for the fact that unless you are really good with a torch, you run the risk of gouging the frame rail which will cause irrepairable damage, and melting hoses and wires also. The zipcut method has dangers all its own....if you are in a tight spot and flex the blade, it will shatter and the shrapnel will cause injury. There are going to be bolts that are unaccessable with a zipcut blade.
So...your job is to remove all the bolts holding the differentials, springs, torque arms, air valves, air ride leveling valve (be extremely careful that you dont move the height control arm the wrong way with air pressure in the tanks or it will cause the bags to overfill, causing who knows what kind of damage) crossmembers, mudflaps, hose and wiring support brackets, etc. Unhook everything.
Measure from the front spring hanger forward mounting bolt, marking the C amount which will be the attachment point for the front axle on that side. Do the same for the other 3 spring hangers.
Using the holes in the framerail that were drilled for the spring hanger, make a template by holding a stiff piece of cardboard up to one side of the frame, and spraying a bit of spraypaint at it from the other side, marking the holes. Do this for all items which bolt to the frame, keep track of what is what, and what direction they must face to match up.