OK, first things first- - - - - - -Can you back off the throttle stop screw any more, to close the throttle plates? I'm assuming you can't, or you would have done that already. Some of the Holley carbs have holes drilled in the primary throttle plates to allow more air to flow without uncovering the transfer ports where the midrange mixture system kicks in. If so, you need to install primary throttles without holes in the plates.
You can slow the idle down by turning the mixture screws in enough to lean it out, but this might cause undue roughness in the idle. With the vacuum gauge connected to full manifold vacuum, turn each idle screw clockwise until the engine tries to stall, and then back it out only enough to keep it running. Once both idle screws are as lean as possible without stalling, back each one out until the vacuum stabilizes, but don't let it get over-rich. You SHOULD be able to slow the idle down with the throttle stop screw, unless the carb is just too big for the engine. If you've got more than a 600 CFM carb on that 283, the carb is too big! Your dieseling problem on shutdown is idle speed, plain and simple! Do whatever it takes to get it down to 550 RPM or so, and the dieseling will stop. Also, the vacuum advance should receive NO vacuum at idle!
Jerry
