Six pack power valve....

My 70 440-6 RR had an idle solenoid, that dropped the carb closed when the key was off.

Things that help to worsen "dieseling" or "after run" are too high idle, too LEAN idle, vacuum leaks, retarded timing, and running way too hot... With a bigger cam, getting the throttle blade too far out of the transfer slots can be a problem. First thing to try is see if it "likes" more advanced initial timing