Turned key off, engine still running
 

And by 'running' I don't mean dieseling or misfiring, I mean running perfectly normally as though I had not just turned the key off and pulled it out of the ignition.

Disconnecting the ignition (I) wire from the solenoid stops the engine. If I reconnect the wire to the solenoid, it cranks right up as expected but it still won't turn off with the key.

What's your diagnosis gang?

thanks

I did a little more troubleshooting during lunch today and discovered a couple other pieces of information.

1. When the engine is running but the key is turned to the OFF position, the rest of the electrics turn off (all dash gauges, fan AC). However, the radio does not turn off. This is interesting because it is a recently installed head unit (about 2-3 weeks ago)...it has been working as expected since install and it would power off when the ignition was turned off. Could be completely unrelated, but it does represent a recent change to the electrical system.
2. I took it around a 5-mile loop near our house to see if anything else was acting weird. The alternator charge gauges continually fell as I drove to the point where it was starting to misfire and I barely made it home.

hopefully that's enough to give you guys the clues you need

No ideas? Visiting my old man this week though ((electrical engineer) so he's going to try to find a schematic so he can maybe figure out what might be the culprit...any ideas to help us narrow it down?

Replace the ignition switch. I just recently had the same issue with my 1966 convertible. Bought a new ignition switch, installed it and that solved the problem.