For what you are doing and going, I would bet ANY of the 1500's with a turbo motor, be it gas or diesel will pull a 7000 lb trailer just fine. Better yet or at least on par as a BIG V8 gas motor at elevation.
The GM 4 banger has IIRC about 320hp and 350 lb ft of torque with an 8 sp trans. 1st is 4.3-1, with 3.42 or if you can get 3.73 gears, should not have issues starting on hills etc.
Not recalling the HP/torque specs for the GM 6 cyl in the 1500 trucks, but I m recalling an 8 or 10 sp behind it. Mid 10-30 mpg.......Could be a good local driver, even if a bit slower on grades, the mpg may be worth it.
Fords Eco boost, especially the 3.5, with the heavy duty payload pkg, probably the BEST truck all around from an MPG, HP/torque, a turbo so the Sierra elevations should not hurt it power wise like you 7.3 in the EX.
I really doubt any of the newer 1500's are bigger per say than an WX. ford made it to be bigger than a GM suburban on purpose. While they will be smaller, not really "SMALLER" per say, talking inches, like no more than 6 maybe 8' length width or height! Altho a crew cab with a 6.5' box would probably be a bit longer.
Good luck on your choices.
Marty