I have a 557 in a Dragster, the only thing that I changed after taking it out of a ranger that I use to drag race is the headers now when I shut off the switch to kill the motor it wants to keep running with no fire to it like a diesel.

 

Welcome to FTE.

Gas engines "run on" or "diesel" because there are either contaminants or deposits or surfaces in the combustion chamber that remain hot enough to ignite an air/fuel mixture. In carbureted engines, as long as the engine is turning, even with the ignition off, air/fuel mixture is drawn into the engine. Many carb'd engines use a dashpot to slow the closure of the throttle to prevent stalling, and later on electric throttle solenoids to keep the throttle open or control idle. Both allow the throttle to close more completely after time or once power is cut.

So, your engine probably has a high idle and enough of a throttle opening to keep air/fuel going even when shut down, and you have some hot spots to ignite it. Being a "dragster", I bet it's jetted rich and that will add to the problem.

 

Dieseling means the engine is providing it's own spark thru heat, as 85e150 points out. So you have to cut off either the air or the fuel when you cut the ignition. Cutting off the air will be much easier by adding a solenoid to hold the throttle open to your desired idle speed only when the ignition is on.