[dsraven]

I would say assemble the whole box and have it ready for install. inspect the stake pockets for a spot where a lifting device could be used to lift from each corner. then jack the box up an blocks tall enough to clear the truck tires as they back under and fab some sort of stand to hang the box safely at that height using the stake pockets as lift points. then back the truck under it and jack the truck up to meet the box.
a few years ago i removed the frame and driveline from a gmc envoy. i jacked the whole thing off the floor high enough so i could roll the spare tire under it. this ensured the truck tires could roll away without hitting anything. then I built a makeshift stand on the front that spanned the width of the truck so the frame could be rolled out from under the body, supported at a point higher than anything on the frame or tires. the rear is fairly straight forward and I used my engine lift to support the body at that end. the front I used a long 2x4 laid flat on the ground which was a lot wider than the body. on that i built a structure that would support the front of the body at the existing height and not allow any side to side movement or fore to aft movement. I used a long 4x4 under the body from side to side, at a point high enough to miss any part of the frame or tires when the frame could be rolled out from under the body, to sit on top of the structure i built on the floor. then unfastened the body mounts etc with floor jacks and jack stands under the frame, in steps because it had to come down from a pretty good height plus I only have 2 floor jacks, and lowered the frame away from the body slowly. when done the frame could be rolled out from under the body easily, the hardest part was rolling the wheels over the 2x4 laid on the floor, and this left the body sitting up on the engine stand at the rear and my makeshift stand at the front. you could do this in reverse in worst case scenario. of course if you have a gantry, a tractor with a bucket lift, a behicle hoist etc etc this would make the job a lot easier. hanging the box from the roof trusses has also been done. spanning several trusses for each hanger of course to spread the weight and using a come along on each corner of the box.
lost of ways to get it done, you could leave the steps off for access to the frot of the box cross sills and lift using a long 4x4 under there, sitting on saw horses, and using an engine lft at the rear.
anyway, when you get it figured out, post up some pics. I suppose the way to get it done without scratching paint is to assemble it on the truck a piece at a time and many trips under the truck.