The reason it came unassembled is that the guy you bought it from screwed up and couldn't get it together and passed it on to you.
The dimensions you seek are probably not to be found except by someone on this site doing the work and passing the info on to you. That's where you're lucky; these guys are very very helpful!
Someone here may even have attempted or succeeded in doing what you're trying.
In general it is easier to use the old car/truck frame and add more modern components to it than to rebody a modern frame. Trucks are easier since the floor area is relatively flat and the bed sits on blocks anyway, but you still have to contend with wheelbase, track, and manufacturing the braketry to hold up your front sheet metal properly and so on and so on.
Best of luck!