Your problem is the altitude- - - - -at 10K feet the carb is running WAY too rich due to decreased air density. My 59 Burb with a 283/3 speed setup used to climb Sonora pass and then go uphill to Leavitt Lake with no problems at all. Definitely DON'T get tempted to increase the main jet size. That will make the engine lose even more power. A Zenith carb with an adjustable main jet might help, but getting the adjustment right every time you start climbing hills will be difficult. Too lean, and you'll start burning pistons and/or valves, and neither one is cheap for GMC engines. What you probably need is some more gears, like a under/direct/over setup with a 3 speed Brownie between the transmission and the rear end. That would let you keep the RPM in the best power range without losing so much road speed. A 270 running at its HP peak should flatten those hills without much of a problem. Those engines won WW II in a whole bunch of different military vehicles.
Jerry
