Is this a one time thing or are you in the resto business? For single one time use, I think you could come up with a frame rack made from a couple wood LVL beams. They are strong, relatively inexpensive compared to steel, and would take the weight of the body. If all you want to do is add wheels to it, roll it, and provide body dimensions and frame alignment spots this should do the trick. I would think you could get factory tolerances or better with this solution.
Some of these home spun, budget friendly, DIY solutions can get pretty good. I recall in a past post someplace Goody coming up with a tilting body rack made of plywood. Thought that was pretty ingenious.