Stall power 79 Sierra Grande 4x4 half ton

[Gear Grinder]

I can't figure out why I don't have any power taking off from a stop.
It won't even spin the tires in gravel. Going up a small hill from a stopped position is imposable, unless I shift the transfer to low. My engine timing is dead on, the vacuum advance is working ok and is connected to direct intake manifold vacuum. I installed a new converter thinking it was bad. No help. I've tried changing the timing with no results. I've changed carb's with no results. With the brakes on, pressing the accelerator, the engine revs to about 1500 RPM and wants to die. If I press the gas pedal any further the engine does die. After I get moving the engines seems to have normal power and accelerates well. The engines revs well in neutral, taking the throttle quickly with out hesitation. The engine doesn't overheat and gets about 11 miles to the gallon, has new plugs and wires. The gears, tires and rear-end are original. I'm just plain stumped. Any suggestions would be greatly appreciated.