With the engine still idling high (1900-2000 RPM) with a prop, I decided to tackle the issue. My choices appear to be either significant shortening of the throttle arm, or moving the throttle stop forward to allow additional arm movement. I had some 3/16" bar, so I fabricated an additional spacer. I need to get a longer bolt as a result, but the idle dropped to a more reasonable 800-900 RPM. I can always modify the arm later if necessary.