In order to drop the voltage to the coil primary by approximately 4 volts, you're going to need a resistor measuring 1-1.5 ohms and capable of flowing 3-4 amps of current. That's going to be a pretty heavy-duty resistor that develops a lot of heat. Stovebolt points can only handle current flow of 5 amps or less, and 3.5-4 amps is a lot better. With the engine running and the charging system developing 14 volts or so, the ideal voltage at the coil + terminal is about 9.5 volts. If an original-equipment ignition resistor isn't available, I'd suggest running four 5-ohm resistors in parallel, with as high a wattage rating as possible, mounted in a position where they can get good airflow from the fan for cooling.
Jerry

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