About 20 years ago I redid all the sheet metal on my 3800 PU. I unbolted the box assembly minus fenders, jacked it up and drove out from under it. It immediately crashed to the ground because of dodgy sawhorses and uneven ground on a hill. I couldn't have maneuvered the assembled box into position to back under it for the above reasons. So, I assembled it, already painted, one piece at a time on the frame. This worked out fine and I needed help only to position the sides and get a couple of bolts started. didn't scratch anything. All that aside, I didn't get the cross sills exactly centered but I have been able to run 255/85x16 tires anyway though the tire on one side is closer to the box.
BTW I couldn't get cross pieces for the 1 Ton so I had to weld in repair sections. No problems in 20 years.
