Exact same thing happened on my 69 340 a few months ago. The car started and there was a horrible grinding noise. I turned the key to "OFF", but the starter kept spinning. I had to pull a battery cable. I posted a thead, and got the same ideas as you. I ordered a new starting relay. To be honest, I never got around to installing it. It's still on my workbench, but that started problem has not happened since. I hate it when something happens like that. It only happens once or twice, and that makes diagnosis tough.
