You know they say that it is a 12 volt system. The Alt puts out 13-14 volts. The car battery will start the car with as low as about 11 volts. If you are lucky.
I think if the coil has the internal resistor, then you can run "battery" voltage, whatever it maybe. 12 to 14 volts.
If you have 12volts coming from the plug that is attached from the firewall (with the KEY in the START Possision) and not out of the wire at the end of the harness (@distiburor), Change the Harness and use the coil with no resistor. The coil should work with the 8 volts in RUN (key switch)
Looking at Johns Mustang Web site. I think it is the $35 FIREWALL TO ENGINE GAUGE FEED. I Did Not spend $450 to fix this problem.
Call the Supplier you deal with and tell him you need the harness from the Distributer to the firewall.