I wasn't sure from the OP if he was talking about hauling a trailer tank or having a bed mounted one. You are right, that makes a world of difference in what you can get away with. Ditto with the terrain. With the mud and slush you must deal with on the ranch in Dakota, the dually 350 would be a huge plus with a bed mounted tank.
FQ13
Who said anything about a dually? Duallys are a pain, more stuff to break. They are best for long-haul trailers, better stability with a loaded trailer, but not for rough or wet conditions, where they actually detract a bit.
I have a permanent 5th wheel ball in the bed, when I hauled water, I had to put the tank on a pallet. Fun with 3500 pound of water on a wooden pallet in a 1-ton truck.
Never had a problem - except for the 1 time where I drove it all the way from town (15+ miles) after filling it up with water, then let ex-GF run it into the pasture as I closed the gate. She ran it past the stock tank, up a small rise, and as she backed it down the tank slid off the pallet and rolled off the tailgate doing a slow-mo somersault over the valve/bung. I hadn't strapped it down, you see, but it even stayed put on the hill away from the water filler.
No damage to the tank, valve or bung, lost all of the water though, only damage was to the tailgate which developed a permanent squared-off U shape. The thin wire straps that held the gate to the pickup did not break, so the metal tailgate bent!