My trailer is about 6500 lbs. My 7 year old truck with 3.92 gears and an 8 speed, has 395 hp. I haven't towed in Colorado, but routinely tow up and down various mountain passes in the west. I can maintain the speed limit anywhere, and still have pedal left. Of course, the RPMs are occasionally up there quite a ways, but the engine is singing, not screaming. On the flats, I'm usually in 7th gear, sometimes 8th. The trailer is easily within the capacity of the truck and tows very well. I can't imagine your GM being much different. If you have issues, unless your truck is genuinely wore out, you should be able to correct what ails it easily enough. Unless you intend to upgrade the trailer (or carry additional payload) in the future, an HD is pure overkill and a waste of money. JMHO of course.