Assuming 3% horsepower loss per 1000 feet of elevation change is linear and that your 261 has 170 HP at sea level, it'd still have 62.21802 horsepower if you can find a road that goes to 33,000 feet Tom. But you'd probably have to change carb jets a few times on the way up. 
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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