Maximum length for 1500

I see people towing similar rigs all the time, but if you are going to do it, I would at least put LT tires on it an invest in a good anti away setup. The biggest danger on the mountains is having to stop hard on a curvy downhill grade, and good sway control and stiff sidewall tires can be the difference between safety stopping and being pushed into incoming traffic. I think you can make that rig fairly safe with the right setup, but you will have a much more comfortable experience with a 2500.