Yes, it is possible the ballast resistor has changed value over time. The '72 service manual indicates the correct value should be 1.6 ohms. This is at ambient temperature (68 degrees F). If you just ran the car the resistor will still warm and will measure too high. The voltage measured at the positive terminal of the coil will vary with engine speed, points dwell angle, and other variables. As I recall my car ('73) was closer to 9 volts at idle. Before chasing ghosts or your tail, replace the coil (you already know it is bad) and check all the basic stuff: points condition, dwell angle, primary wiring, etc. If the car still is not running right look at the less likely stuff.