Three things have to be right to get good performance- - - - -fuel, spark, and valve timing. Unless you've had the camshaft out of it recently, the cam timing isn't likely to be an issue, but it never hurts to check. Pull the valve cover off, and watch the valves on the #6 cylinder. The exhaust valve will close, and the intake will just begin to open as the #1 and #6 pistons reach the top of the stroke. The distributor rotor should be pointing toward the #1 plug wire terminal as the #6 valves overlap, also, so it's a good way to confirm you've got the distributor timed right at the same time. If the dizzy is timed a tooth late, it's still possible for the engine to run, but poorly.
The most likely problem is going to be a fuel-related problem. If the plugs are black and sooty, that indicates the mixture is too rich. Check the carburetor float to see if it's got a leak. A heavy float will allow the fuel level in the carb to get too high and spill over into the intake, causing an over-rich condition. That will produce the stumble and stall you're experiencing, and a major power loss. Look down the carb throat at idle, and see if fuel is percolating out of the nozzle in the venturi, about halfway between the air horn and the throttle plates. If so, you've found your problem.
Jerry
