The only thing that lets an engine run on after the ignition is shut is getting air from somewhere (as said). The idle speed is too fast, you have a vacuum leak, power brake diaphragm is bad, etc.
If all else fails, a curb idle solenoid from a late engine can easily be adapted to your existing throttle stop, powered through the ignition to be hot with key on. Key on activates the solenoid plunger and raises the idle speed slightly (easily adjustable for the exact speed for a good idle), and key off drops it back to nearly closed to prevent dieseling.
Any brand, any carburetor as long as the voltage matches your system.
