ballast resistor resistance?

The main thing is the voltage at coil+ (red lead) with the black lead on BATT-. You should read ~8 VDC (average) with the engine running, if points or the OEM Mopar box. If lower, you have excessive drops in the connectors, key switch, or ballast.

When you measured 12 V on both sides of the ballast was the engine running? If not, you did a bad thing - you left the ignition in "run" with the engine off. I have read that can burn-up your points or Mopar box (black epoxy runs down the inner fender).