Larger isn't for everyone but one thing I love about a diesel in the mountains is the power and engine braking. My experience with mountains is the California Grape Vine, Alpine east of San Diego, the 17 mile grade in the desert heading to Las Vegas where air temps will be 90+.
None of these require but one gear shift out of overdrive and going down hill leaving it in overdrive even at 6% grade it still slows by itself without use of the brakes. Can only imagine a Diesel truck that isn't 27 years old would only be better ;)