I think oldmanrick summed it up best. I drove big rigs for some years. I once had a 76 short bed with the 225 and auto. One time (note ONE TIME) I towed a full size Ferguson tractor with a loader bucket and backhoe up into the Sierra Nevadas. Way heavier than a car. I was young, it was kinda foolish and abusive, but it made it. The big weakness was the auto, but you have that solved with the 4 spd. Even a high-cube Cat engine in a Kenworth will only pull so fast uphill. The thing to remember (unknown to many RV haulers and such) is that you have to drive it like a truck, not a sports car. Drive ahead, don't lug the engine uphill, don't push it beyond its capabilities, don't try to keep up with the Camaros.
I also STRONGLY suggest, if you'll be pulling trailers with cars on them, for $600 you can afford a little extra to install an electric brake sender, and get a trailer set-up with electric brakes. Surge brakes on rental trailers are next to useless. Regardless of what engine you have, don't forget when you get to the top of the hill, you have to go down, and a trailer and car edging towards 5,000 lbs pushes hard on a truck that weighs less. I've had some tedious uphill climbs, and I always follow the rules (in most states) more than 5 cars behind you, find a place to pull over and let them pass. But I never had a scary uphill ride. I've had some very scary downhill runs, in big rigs with malfunctions and little rigs not well equiped. Be safe, and enjoy that $600 deal!
