I expect the truck will be really fine with the new trailer behind it. I will be quite a bit lighter than my toyhauler is. I know we are overloaded, but I guess that is what happens sometimes when you live in these things. The problem with the Ford was mainly only when the trailer brakes failed. I don't know much about the mountain passes out in the eastern US, but I can say the highways we drive out here a lot are steep and narrow with a lot of switchbacks. My F-350 pulled good as long as you could keep up your momentum, but when you come out of a switchback on the side of a 7.5-9% grade with 20K behind you it would only get back up to 25-30 mph. My 05 Cummins dually with some mods was about the same. When we haul firewood is when it got nerve racking most of the time. I don't know what the weight was exactly, but I have been told green pine is around 3200 Lbs. per cord. With six cords on a 6500 pound gooseneck it could get interesting coming downhill. Especially since we have thousands of miles of dirt roads (oilfield roads) in many of the areas we camp/hunt/cut firewood, and the passes on those are 15-18% grades in some case. Going uphill was just a patience game and not to bothersome. I do wish Ford had still offered the 6 spd. manual, I think that would have helped. I have always liked the cummins, and still own a 93', the 05' and 07' I had just kind of made me leary of them.
When I was looking for a new truck to replace the F-350 with, I was caught between the Dodge 4500-5500 and the F-450/550. The problem was they are so dang expensive! Even a used one is high. So I started looking at a more full size MDT (freightliner, international, etc.). I found them to be even more ridiculous in price, and not really any more power than the Dodge or Ford. So I finally went over to the escapees website. After a little reading I decided to see what I could find. I found many fairly clean class 8 trucks from $15,000-$35,000. I finally settled on mine for $17,800. I sold my ford for what we owed and the new bed, and put another $2,200 in it. It has been good for the boys to have the sleeper on trips, and my wife and Sassy have plenty of room for whatever.