Without going too crazy slightly modding a 5. 9L cummins is just fine. The engine will probably out last the truck even if you cut its normal life in half. The thing that you have to watch is the stock clutch. I've talked to a few guys that have changed them out with low miles and not even crazy power yet. Match rpms when downshifting, (espescially with an exhaust brake, clutch wears both ways; starting and slowing a load) and keep torque under ~700-750 ft lbs. My brother has an '01 six speed with about 300hp and 700ft lbs w/ bd brake and its pulled a lot. He has 278,000 kms on it and still original clutch.