I'm have no clue on how to adjust it, but I did square up a '79 Trans Am chassis a while back. To make it easy for me. I used a plumb-bob and string to drop points to the garage floor. I used wide painters tape to mark the points on the floor so they showed up clearly. Once I had all the important points marked on the floor, it was much easier to measure from point to point on the level floor using a tape measure. If you use two different tapes or yard sticks, verify they read the same. I have two that don't agree with each other.
Drop points down from the kingpins and rear axle housing so you can check wheel base side to side and criss cross to confirm it's square or not. Have both axles on jack stands so the truck is setting level. Just be sure you use the same exact spot to drop points on each corner
Joe
