318 has developed a miss?

By "alternator guage", do you mean the stock ammeter? If so, I assume it is flikcing towards the D side (discharge) when you have the miss?

After your tune-up checks: A spark around the distributor or spark plug wires would not likely cause the alternator output to drop, as the ballast and coil isolates the spark area from the car's power system. It is more likely the other way around: some erratic wiring issue is causing the voltage to the ignition systm to drop and cut out the ignition momentarily.

I would start with a voltmeter and attach it to several points at different time and monitor to see what it does when you get the miss; you would need an meter type of voltmeter not a digitial type, to be easy to read the fluctuations. I would 1st monitor the dark blue wire connection to the ballast resistor; this is your ignition power source when running and also feeds the voltage regulator. An erratic, faulty ignition switch would be detected by this.

Also, have you disconnected, and inspected the bulkead connctors?