Check to see that your wires are secure on the ammeter posts. I loose wire there will cause your symptoms. Ask me how I know. The current owner of my '50 failed to tighten the nuts back there...
That loose wire was still making enough contact to keep the coil happy, but when I load that weak connection by turning on the headlights, it tripped the breaker in the headlight switch while at the same time robbed the meager amount of juice going to the coil, causing the engine to die.
That is what I am guessing is what happened. It needs to be made clear that I am not that convinced that electricity is real at all. The only evidence that I have ever seem to give an indication that electricity does actually exist is when I let the smoke out of a wire.
In any case, once I tightened the nut on the ammeter, the issue went away.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission