DO NOT USE the ignition coil terminal. The voltage is usually around 5-6 volts once the engine is running. This is deliberate and the coil is designed to work in this way.
Early Mustangs are very poor electrically compared to modern cars.
Do as Wicked93 suggests. A thin wire from the ignition switch to operate a relay. The relay switches a heavier wire from the battery +ve (or the hot terminal on the solenoid) to a fuse block. Take your choke wire from the fuse block.
Use a 'heavy duty' relay capable of switching 40 amps or more ( I did 100A on my '69) coz you never know what else you may want to power from the fuse block.
I ended up rewiring my headlights, horns, spotlights all via smaller relays, from the fuse block. Less voltage drop , less chance of burning out under-dash wiring.