I like your thoughts, but you'd have a very hard time putting any other body on the 5 ton chassis. The hood alone is over 7 feet long to cover the engine. The 5 ton 6x6 chassis is also very well built. There's more than just rockwells under a big frame there. They have a very nice full synchro spicer 5 speed in them and a huge transfercase. Air over hydraulic brakes, on board air system pre-plumbed with connections all over, oversize drivelines and U-joints.
The DT466 is a great engine, but it's at home in a 10K pound empty truck, that's what it was designed for, it's a medium duty engine. The 5 ton chassis is the only thing I can think of that actually fits a heavy duty engine (The 855 Cummins) in a juice brake medium duty chassis. The DT466 and 855 Cummins are apples and oranges. 466 cubic inches to 855? The NA 855 that comes in the 5 ton trucks is the equivelant to a healthy turbo 466. The Cummins big cam 400 is an instant 400 HP with rediculous torque, unit injection and real Jakes. Many out there are actually running more than double the 400 HP.
Also, the 5 ton 6x6 weighs more than 10K, but when you cut it down to a 4x4 and use the utility trailer for a bed it weighs in right at 10K.