Tell me about it. I'm at my grandparents house putting up a fence for them around there property. I hauled our trailer and tractor over, about 7000 pounds I think. Flat ground I was holding 62mph real good. Pulling up the hills and grades to there house though...was iffy. They live on a steep road with turns, so you come out of one corner and instantly hit a 8% grade with 20mph speed. 3rd gear is too tall, and 2nd is to short it seems.
Just gotta live with it. All I know is I can maintain 65mph up hills loaded with what I was hauling as I was in a real sweet spot in the rpm's. 9psi of boost, 1100* EGT's and it pulled it
Like said by Chris, a turbo will be your best bet.