I picked up 4406 today from the body shop. I swapped the cables of spark plugs 5 and 8 then 6 and 7. With the old carburetor and the points ignition it starts like clockwork, no more backfire and a stable idle. It's surprising how much better it works when the spark plug wires are all plugged into the right place.
I put the new carburetor back in and it runs properly although it still needs to be adjusted.
Tomorrow I replace the breaker ignition with the Pertronix.