340 cranks over forever when warm

I had the same problem with my 69 340. Do you have a phenolic spacer under the carb? You need some thing to isolate the intake manifold heat from the carb.

Without the spacer or a thick gasket, the heat will boil the fuel in the float bowl. A minute or so after you shut it off, if you look down the carb throat, you may see the fuel dripping from the boosters. If so, heat is the problem and you need to insulate it from the carb.

Hope this helps.