Matt,
I would look for a big engine (the MBE would be the smallest) and a 10 speed autoshift or 12 speed freedomline. You need both to be happy in the hills of the southwest.
I was basically in the same situation and here is why I ended up with a Pete.
Living in northern AZ and the need to go up the nice hills, like 89 up flagstaff, the grades up and down Laughlin. Entering end exit the 15 in the middle of the mountain pass grade (Nipton road) and many times running up and down the I-40 between Kingman and Flagstaff and I-17 down to Phoenix the big priority was the engine.
engine I ended with the Cummins N14 because it has 1550 lbsft of torque, but also the 14 liter (855cid) together with the Jake brake gives me enough braking HP to slow the load safely down .
Gearbox, I would like to have an autoshift, but there are no Pete 379 with such a thing. So I got the next best thing, the super 10. With your load in the hill I would get at least 10 speeds. the allison auto is missing a gear between 5 and 6. Also you will not find many OTR trucks with allison, not even the fleets use them ... that may tell you something.
Then truck itself, I like the T2 and Pete 387 and we looked a long time to get one of them. We decided against them during our move from the CA highdesert to northern AZ in the middle of summer with our International 4900. I know now exactly why there are so many W900 and 379 with big visors on the I-40. The small cab together with the visor keeps you cool.
Next is the speed, AZ legal 75mph (maybe soon 80mph), the same for NV, Utah I was looking for a conventional and stayed away from set back axles. the 379 goes straight as an arrow ... but also the lower cab compared to the T2 and centurion class freightliner gives a better/safer feeling in turns and more comfort on not so well maintained backroads.
then finally parts. For a cat or cummins and a eaton fuller roadranger gearbox you will find parts anywhere. MBE or Volvo can be a different story.
hope that helps in your decision
Thomas