I can tell you from first hand experience that doing mountains takes a LOT of horsepower.
I pulled my cargo trailer ( 8,800 lbs ) with my 1976 Newell with a good running 6V92
A 6% or greater grade will have you down to about 10 mph in First Gear. Not a pleasant situation.
If you plan on pulling something heavy, You absolutely need all the power that you can get. You also should have a good Jake Brake or Retarder.
Getting up the mountain is one thing that wears you out but that downhill part will absolutely scare the heck out of you