The spark plug heat range is not much of a player when having engine ‘run on’.  The engine not stopping cleanly when killing it is more related to the gasoline octane level being too low and/or too much throttle opening on the carburetor.  You can try setting the idle speed lower and see if that helps.

Lorena, Texas (South of Waco)

I’m using Autolite #46 plugs gapped to .034 in my 312, it runs great but every once in awhile I’ll get a little run on. I’m familiar with the carbon, fuel grade etc causes but I’m curious if anyone has found a plug that is good at combating it?

1955 312 T-Bird Warwick, NY