I will not tell you what you need, but you must understand even if you feed the biggest gas engine you will hear a lot of the mountains will slow you down
Point is if your load is to heavy you might not want to run WFO all the way up the hill to avoid overheating. As soon as the temps starts up, back out of it, let speed drop. Will others pass you? So what! A mile long climb, at 60 MPH, takes you a minute. Same hill, at 20 MPH, takes 3 minutes. Traveling the country you will find very few places that the biggest engine will run 60, and the smallest will pull down below 40. If the manufactures don't use bigger brakes with bigger engine the bigger engine not worth that much in mountains.