The problem I had with run-on was that the idle adjustment screw held the throttle blades open and allowed the engine enough fuel and air to run. Once I closed this off, no more run-on. You state that the engine will not idle in gear if the "park" idle is less than 1000 rpms. Perhaps you could adjust the idle speed in gear with the timing and back the idle screw all the way down. The AC solenoid method would work too. Either of these would be easier than a converter swap.