I am curious why you are not setting the dwell at 39 degrees. Are the points brand new? If not I would start with new points and condensor and set the dwell at 39 degrees.
If the stumble continues, you might have an internal leak in the air horn due to slight warpage (some Rochester B carbs are notorious for that).
If you look down the throat of the carb while the engine is running and open the throttle a bit you can sometimes see fuel making a wet spot on the edge of the gasket between the air horn and the main case below. It will get wet when you open the throttle, then dry right up when the throttle is closed.
The stumble comes from fuel going where it does not belong.
Carl
