John, I'm always learning from this site!
I did'nt realize the cloth covered wire was a resistance wire!
As JohnZ suggested, pick up the 12 volt source from somewhere else, wiper motor is one possibility, straight off of the battery is another.
There are two primary circuit wires from the ignition switch that connect to the + terminal of the coil; one wire for applying a full 12 volts for starting and a second "resistance" wire which reduces the voltage to the primary side of the coil when the engine is running. The wires to the distributor and the tach lead (if you have a tach) connect to the negative (-) terminal of the coil.
When the ignition switch is in the start position, the ballast resistance (or resistance wire) is not in the circuit, and a full 12 volts is applied to the primary winding of the coil for starting. After the engine starts and the ignition switch is in the "on" or "run" position, the ballast resistance wire is now in the circuit and the other wire is not. The ballast resistance drops the voltage from 12 to somewhere around 7-9 volts, enough for running while saving wear by reducing arcing across the points in the distributor.
Attached pic is typical. It shows a ballast resistor in the "start" circuit. Sometimes GM used a ballast resistor and in other applications GM used a special resistance wire of a given length to create the required resistance to drop the voltage from 12 to 9 or so. Not sure of other years, but my '69 RS has the resistance wire which is easily identified by the cloth like material that covers the OD of the wire as described above.
On both my 66 Chevelle as well as my 69 RS, I have made the conversion to the Pertronix Ignitor electronic ignition with their flame thrower coil and all works just fine with the resistance (ballast) wire in place. The Petronix electronic package is designed to operate between 8 and 16 volts. The instructions clearly state: "If your ignition system presently has a ballast resistor, do not remove it." I would recommend that you do not modify the cloth covered resistance wire at all. As mentioned above, when the engine is running you do not have a full 12 volts at the coil due to the resistance wire being in the circuit. However, you should have enough voltage for the Pertronix package, especially if you have changed out from the stock coil to their "Flame Thrower" coil. I have checked my 69 and I usually have 9 to 9.4 volts (Pertronix wants a minimum of
at the + terminal of the coil with the engine running (resistance wire is in the circuit).