I have a '75 GMC and when I had similar problems it turned out to be a sliver of RTV that had been sucked into the carburetor when i had the air cleaner off. The sliver of RTV was plugging the venturi on one side, so the fuel mixture was way off. Once I took it apart (its an Edelbrock carb) and cleaned out the debris, the truck ran properly under load.
You may not have exactly the same issue, but I am almost sure your problem is also fuel related. Your idle circuit is working fairly well, which is why it runs at idle, but once you transition to normal cruise you don't have any power.
When I was troubleshooting my truck the Edelbrock tech suggested holding a mirror at an angle and using a flashlight to look down into the carb while the engine was running. When I did that there was an obvious dribble of raw fuel on one side, which was due to the plugged venturi.