Don't forget to factor in the gas engine will last about 150/200k miles and the diesel engine will double that. All you have to worry about is will the truck components and body last that long.
Hey Bert, my Dodge 2500 is a 4x2, reg cab so probably weighs about 1000lbs less than your 3500 D/A. I just stick it in 5th and hold till it gets there. I have to back off the throttle on most 6% grades due to high rpm. I've only been on one grade ...