It all depends on your penchant for speed, and your patience. I'd never run one but there's a tunnel just to the west of me at 11,000' where the speed limit is 65 mph. If your mountain passes are slower, maybe not as big of a deal. Later V10s and the 6.0s will get the 5r110 though which adds an extra gear and makes the 4x4 conversion easier as well.
My first E-350 was a V10, I flew to CA to get it and drove it back to CO. Expanding your search helps too.
'06 EB 350, Advanced 4x4 (soon to be MG), 6.7 Cummins + Allison, 18" shuttle top. Bright yellow. The rumble bee.