Only info on that coil that I could find was this picture of one for sale.  It states it must be used with an external resistor.  Isn’t that telling us it must be used with the pink resistance wire, meaning it runs on nine volts?   I would think a coil designated to work with a twelve volt system would be more compatible for long term success with a Duraspark system running on 12 volts.   

Typically, you would bypass the pink underdash resistance wire when a a Duraspark is wired into a 1969 Mustang.

I searched for specs on that coil, and the closest I came to finding anything was that picture.

It will work better with full 12 volts to the module.  Also works best with both power wires connected as in the diagram. That way it tetards timing during start for easier cranking.

Sounds like you have the correct 12 volt coil.

If you run a copper wire parallel to the pink wire under the dash, it allows use of the factory plug through the firewall. The pink wire ends before it goes into the plug.

MS wrote:

It will work better with full 12 volts to the module. Also works best with both power wires connected as in the diagram. That way it tetards timing during start for easier cranking.

Sounds like you have the correct 12 volt coil.

If you run a copper wire parallel to the pink wire under the dash, it allows use of the factory plug through the firewall. The pink wire ends before it goes into the plug.

I may be misunderstanding but the power supply to the Duraspark module and the power supply to the coil are 2 different things.  The Duraspark module needs a full 12V to function properly.  But all it does is switch the negative side of the coil to ground just like the points would do.   

The voltage to the coil + terminal is dependent on the coil design and independent of the Duraspark module power requirement.  The module works just fine with a coil that requires the resistor wire as long as the module has it's 12V.
 

Boss980 wrote:

Looks like this answers my question 

That's an old diagram and although it will work, it won't work as well as having the ignition module connected to a full 12V.  The current diagram is at https://www.mustangsteve.com/conversion.html

Yep

Old diagram.  Made before the guys on this forum fully educated me about the Duraspark requirements.  And that was many years ago!!!  Just shows it is impossible to recall stuff once it is posted on the net.

Module and coil works with only 9 volts but is actually designed to use 12.
So, eliminate the pink wire to the module so both the module AND the coil are run on 12 volts.