Engine run on with A/C on

[walker]

I ran across an interesting problem wit my '67 the other day. It has always run for a few seconds after the key was off and removed from the switch. This is not dieseling, or anything like that just normal running. I had played around with fuel mixture and timing to try to make it go away, but it never changed.
Yesterday I had turned it of after turning off the heater fan and it shut right down, so I restarted it and turned the fan on and shut the engine down. It would runs for a few seconds after the key was off when the fan was on, but if I turned the fan off first it would shut right off. Puzzling, the only thing I could think was happening was that the fan was somehow inducing electric current into the ignition system.
Has anybody else run cross this, and what has been done about it?
BTW it has an HEI dizzy in it and has been converted to an internal reg alternator by jumping the wires at the VR (I found the directions on the site somewhere).