67 C-10 won't turn off

[stock67]

I have a stock inline 250 67 Chevy C-10 with 90K original miles on it. I replaced the headlight switch a few weeks ago and shortly afterwards the truck wouldn't turn off. I can turn off the ignition switch off and remove the key but the engine keeps going. I replaced the ignition switch, but that didn't change the problem. I have checked the starter motor for any loose or fried wires and checked the wiring harness for any damage, none found. A few years ago I replaced the stock ignition with an HEi unit, other than that the engine is stock. The wiring on the truck is stock and unmodified. Any ideas of what I can do to fix this problem?