That >>fair amount of built-in resistance.<< amounts to 1.8 ohms in a 6 cyl wiring harness. 1.6 ohms for V8 harness. I don't know about the '66, but on a '67 there is copper wire from ignition switch to the firewall plug/connector. The resistor wire goes from that connector to the coil. Mine actually measures 1.7 ohms.
The stock GM HEI uses less than 2 amps so there is no need for 10 or 12 gauge wire. The ign switch is fed by a 12 gauge wire and it supplies the ign, radio, wiper, heater, dash and tail lights. 10 gauge from the ign switch to HEI is like using a 1/2"garden hose to feed a 3" fire hose.
Kerry, if you are >>electrically challenged<< , then make the conversion simple. Crimp the new connector on the end of the coil wire and plug it into the HEI. You probably will have to add a length of wire for it to reach.
When I originally put the HEI on my 292 about 15 years ago, I bypassed the resistor wire and the HEI module burned out the third time I started the engine. It was a cheap flaps aftermarket module and I believe a voltage spike burned it out. I reconnected the resistor wire and removed the start wire from the "R" terminal on the starter solenoid to make sure it didn't spike the voltage when the starter load dropped.
I changed the HEI to a remote coil HEI a few years ago to give more room for the AC compressor. It is still connected through the original resistance wire. With 14 volts at the battery and 11 volts at the HEI coil at idle. About a 2.8 V drop, measured across the resistor. I've driven cars home when alternators went out and the HEI still worked down to 8.5 Volts at the battery.
Yes, if you are going to an aftermarket HEI or bigger coil it will use more current and will need the 14 volts.
If you want it simple, just hook it up and drive it.
HEI HEI coil Engine 
