Electric chokes are a spring that is heated by the 12V current applied to them. Manifold vacuum pulls the choke blades closed, the heated spring opens them as it warms up with 12V applied. The original carters used a bimetal spring heated by the exhaust manifold. The electrics simply use a rough heating time to open the choke blades after the ignition is turned on. So you need a switch 12V source, not jus momentary.
They reccomend against the coil wire because it can drop the voltage too much causing burned out coils and hard starts. With the alternator sensor wire, the voltage drop can cause the alternator to think it's providing too low of voltage. This will result in overcharging and high voltage.
You can run a new wire directly to the key switch. Or you can replace the ignition to ballast resitor/coil wire with a larger guage to prevent voltage drop. 12g wire would not be a bad idea if you do this. My HEI (DUI) ignitions require a 14g all by itself.
Gary