[ERA 778]

Your problem reminded me of something i read some time ago and it took a few minutes to track it down. Your situation sounds somewhat like the problem described in MSD's Troubleshooting Guide for their 6530 ignition boxes:

"ENGINE RUN-ON
If your engine continues to run even when the ignition is turned Off you are experiencing engine Run-On. This usually only occurs on older vehicles with an external voltage regulator. Because the MSD receives power directly from the battery, it does not require much current to keep the unit energized. If you are experiencing Run-On, it is due to a small amount of voltage going through the charging lamp indicator and feeding the small Red wire even if the key is turned off.
Early Ford and GM: To solve the Run-On problem, a Diode is supplied with the MSD in the parts bag. By installing this Diode in-line of the wire that goes to the Charging indicator, the voltage is kept from entering the MSD. Figure 14 shows the proper installation for early Ford and GM vehicles.
Note: Diodes are used to allow voltage to flow only one way. Make sure the Diode is installed
facing the proper direction (as shown in Figure 14).
Ford: Install the Diode in-line to the wire going to the #1 terminal.
GM: Install the Diode in-line to the wire going to terminal #4.
Figure 14 Installing the Diode to fix Run-On."

You might have just part of the symptoms.

You can download the entire manual from MSD's website.

Hal