that looks different then the later 70s setup or at least what short boxes had. Good thing someone else chimed in or my answer wouldn't have made any sense at all!
I just went up and took some photos of my long box. I think you are spot on. My box is from a 1968 and is the original box. Here are some photos I took of the mounting locations and they are really different from RapidusMaximus' pictures. Just some explanations: The orange paint marks are where the six bed attachment bolts go. The front mounts seem to be small "U" channels bolted to the side of the front crossmember. It does look like the right one has lost its lower lip. The center bolts go through a crossmember (the crossmembers appearing to be square tubing as opposed to the reinforcements that look like a open "U" channel). Unless someone has a 1968 long box Utiline, I think I am going to take RapidusMaximus's dimensions for the rear blocks, assume there is nothing other than maybe a thin anti-sqeak pad for the center bolts, and just make the front spacers thick enough to get a level box using the rear and center as a reference. If anyone else has an idea, as O-Ren Ishii put it, now's the time!
Thanks to both of you for responding.
