[DanEC]

Lippy is correct and that method (dragging the motor rpm down slightly with the clutch before turning off the key) is the easiest way to deal with it if you don't want to lower the idle.

You can also install an idle solenoid on the carb that increases the primary throttle blade opening with the key on but when the key is switched off, the solenoid closes the throttle. This was basically Detroits way of dealing with this issue back in the late 60s/early 70s. They are available for Holley and Edelbrock carbs including brackets but will probably run you about $100.