Old dog,

Welcome to the Stovebolt. When I see a four barrel carburetor stuck on a 6 cylinder engine, I automatically blame it for most problems. This might not be accurate thinking though.

You said it breaks up under load. To me, this means it will come up to full RPM while just sitting in the driveway...no problem at all.

Things fall apart when you accelerate up a hill or when you lug the engine...is this what you meant?

If that is the case, ignition would be my first thought. If that particular carburetor has a circuit that responds to low vacuum that could do it too.

Knowing if the rig runs perfectly except when heavily lugged or loaded is the key to finding the problem. A little more explanation would help with the puzzle.

Stuart