I think you would have to do some serious reconstruction to get the DT466 to fit. I'd also be kinda scared of the stock frame, considering the extra weight of the engine and trans, not to mention it's probably putting out three times the torque of the original 216.
The Cummins may be a better choice, but I'd still box the frame rails or stiffen it up some way.
You may be time and money ahead by just swapping the 46 cab onto a Dodge 1 ton rolling chassis.
