Cummins, 4x4 and "carry a 4,000lb machine on the bed, and a 10,000lb trailer regularly."
That sounds like a 20k+ CGVW.
You have big truck front fenders with the larger opening.
I think buying a running, driving cummins powered 4x4 dually chassis and putting the body on it might be the way to go here. with large tires you will probably have some stick out, which is a popular look for big 4x4 anyway.
Pictures of cummins in '58, '59 shows a large firewall bump is needed and the engine bay is tight. I think I might consider how extensive the body work would be if you center the fenders over wheels, put grill in front of rad location and put cab behind the cummins.
Was there a long nose version of this front sheet metal from the factory?
you have probably seen this. More tire than I'd want but cool:
https://www.youtube.com/watch?v=wZd-U5eZ97c