Thanks @Newton,
Well, we do know the bed is 6.5ft (1981.2mm) long and 57inches (1447.8mm) wide, and the rail angle can be fairly accurately estimated from the side profile. It is just the bed height that is unknown (at least to me). I'll start with a ratio of width to height of the tailgate...
@Newton , Did you happen to find a good CAD file? I'm still looking for a good one to start drafting up my camper design on top of. All the ones I have downloaded from grabcad.com have a tapered bed for the tonneau cover, which is totally wrong since it the tonneau is on rails that obviously...
Cut out dent, weld a new same size piece of stainless steel in place, grind the edges. Possible spray can of protective film. Done. A handyman can do it. Battle scar for the armored personnel carrier looks good.