My father in law has the 6.0 in his Chevy. Great engine, and seems to make plenty of power for him. Its one of the "older" gas engines in heavy duty pickups, but that's because its so dependable. I'm not a GM guy AT ALL so I can't attest to their long term maintenance schedule, but from what I understand its a pretty darn good engine.
I would just be concerned about the abysmal fuel mileage in a 1 ton pulling a 5er with gas. Then again, with the cost savings versus a diesel, you can buy a LOT of gas... so what do I know?