Ditch the ballast resistor, and attach those 2 ballast wires together and there is your true 12V to the HEI coil. What about using a mopar electronic distributor, and firing it with an HEI 4 pin module. Then you keep the distributor as a mopar setup and use a pertronix cylindrical coil. This avoids that dreaded fat cap HEI that is a tight fit on mopar firewalls. I got my 4 pin mounting bracket from www.designed2drive.com