That looks like a pickup cab and box pushed together no? My dad built an offroad racer that way in the mid 70's and the cab and boxsides wouldnt stay welded (kept ripping apart even with a boxed Blazer frame). So he ended up ditching the box floor, inner box sides, tailgate inner panel, and then everything flexed enough to survive. I keep bugging him for his photo album/boxes so I can scan the old pics, but he hasnt came through yet. Currently the cab is part of the motorhome, but with a different clip and custom camper.
I'll keep bugging him about pre-motorhome racetruck days photos.