Cummins 8.3 330hp ?

I have an 8.3 Cummins 300 HP, with a 4 speed allison, its old, a 1992 American Eagle. It had 120k plus miles on it when we bought it. So far I have added 8k. I pull a Saturn Vue which is pretty light weight, can go 70mph down the highway. Slow starting out from stop lights until the turbo kicks in, have not gone up more then the standard 8% grade that I know of, it slows down, but if I can get a clear spot generally goes around semis going up. Now if I get behind one that is going really slow on much of a grade I have trouble picking up much speed. Some of the semis will block both lanes and that is really aggravating to me. I have no idea how it would do in true mountains, hubby says very slow but it would get there.