need advice on new truck

I have been looking at lighter weight trucks for pulling end dumps. My current t600 with a 39' frameless weighs in at 30,800 with half tank of fuel. my door weight is 17,280. I have been looking for a lighter weight truck and want some opinions. I have found some 07 pete 357s with cummins ism 400hp engines in them. I currently have a 475 c15 with a 13spd and 3:36 rears. The petes have 400hp with 10 spds and 4:11 rears. I know the 2 engines are not even really comparable, but the door weights are 13,878. I have also been looking at some small bunk freightliner centuries with 14L detroits. They weigh about 1,000lbs less than mine. Mainly i want to know how that ism would do pulling around 80-88k on a daily basis? The rear ratios are a lot lower on the ism's, will that help the lack of power a lot? Also what kind of life have people been getting from them? These 07 petes have around 710k on them with no record of overhauls that they can find. Also what kind of mpg will that ism get hauling bulk?