When you turn off the ignition does the wire to the HEI go to 0 volts?
If not something is wrong with the wiring to the fuse block or the ignition switch is bad.
The one wire alternator should be connected the battery side (hot at all times) of the fuse block. If connected correctly a one wire alternator cannot feedback to the ignition. Double check the alternator connection. As a test disconnect it and see if the problem goes away.