have you used a 5 ton tractor with its stock setup? I have a M932A1 that has the super singles and winch. I pull a 40' inclosed trailer, and I don't have much of a problem getting it where I need it.
the truck doesn't turn very sharp though, but thats because of the live front axle.. you can't cut the frame to make it shorter because the trailer already doesn't clear the cab by much..
also the military 5th wheel sets up higher, and you would have to have the ramps to get the trailer hooked up, unless you went with a civy hitch, then you could move the hitch back, and shorten it by 1 1/2- 2 feet..
plus with a heavy trailer, you will
WANT/NEED the 6x6 if you are off-road much at all...
in short- I dont think I would trust one cut down to a single axle.. just my
edit* I just read your sig, and see that you have had a M818, and have a M915..