Assuming that the box on the truck is square and not racked then just use a tape measure to insure the studs are parallel to the sides of truck walls.
I.E. for the first stud place the top of the stud at 15 1/4" inches from the top of the truck wall and the bottom of the stud is 15 1/4" inches from the bottom of the truck wall. And so on across with your studs at 16" on center if that matters to you. ( i think since it is a truck that will be moving, bouncing i would opt for studs at 12" on center and lot and lots of cross bracing and fasteners.)
The same applies to how far the studs are from the back of the truck box opening, cut two studs so they fit in the top and bottom tracks ( top plate, bottom plate ) at both sides snugly, fit them into place and then slide ( gently tap ) the top and bottom tracks until you measure that the studs are straight in line with side of the box opening and then screw the top and bottom tracks into place.
It does not need to be perfect but it does need to be the same on both sides.
If the box is racked ( equal measurement of the opening diagonally from corner to corner both ways ) then use a jack and some jack stands or blocking to level the truck. If the box is still racked then you may need to "manipulate" it some.
Personally i would not worry about leveling the truck and just get the box as close to square as you can i use the method i have instructed above.