[Lynn Eades]

If its a six volt coil, and your running six volts, and it says no resistor required, then dont use one.
The original Ford coil is designed to run on 4.5 volts, and it gets this 4.5 volts, by applying 6 volts to a resistor,(in the instrument panel) out of which is delivered 4.5 volts, to the coil.
If you try to run your 6 volt coil on 4.5 volts, your high tension voltage will suffer dramatically, and it more than likely wont even fire.
Later vehicles with resistors usually worked something like this:
With a 12 volt system they would use a 12/9 volt ballast coil.
When the engine is running the coil gets a reduced voltage (9 volts) through a resistor. When the key is switched to the "start" position, the resistor is bypassed applying the full 12 volts to the coil. This effectively increases the spark at a time when it would otherwise be weakened by the load of the starter motor under cranking.