Here's on more thing to consider. Whether it's a 250 or a 350 and a single rear wheel it may still not safely pull a Montana. Many Montanas will put enough weight in the bed that you can't buy tires that can handle that weight. You can guess how I know. If you overloead the tires, buldges in the sidewall and cracked beads are in your future, regardless of what it says on your door.
Dick, Joyce, Diego, Picatso and Gustav
2017 3720 RL, MorRyde pin box
2017 3500 Dodge Maxi DRW 4X4, Pullrite Hitch, IS, Disk Brakes, 3rd AC, Winegard Traveler, Bathroom door mod, Dometic 320, couch for desk swap, etc.