I recommend a 2-lever remote box. If it’s not idling perfect, and/or if the idle is sometimes a bit uneven, you can increase throttle slightly to get a good speed to shift at so the motor doesn’t kill. Sometimes this is handy when the engine is cold to be able to give a bit more throttle for shifting. If you are running on a hydroplane, I’d just use a spring-loaded dead-man’s throttle for safety reasons, and no remote shift, as you can just reach back to shift it.
That’s what I am thinking. I have a dead-man’s throttle and cable tied to a KG7, but I can just as easily make one for the Mark 20. I’m just not sure how the cable would run for the throttle.
1952 Blue Streak "B-Stock Runabout" - 1952 Mercury KG7Q
1955 Feather Craft Vagabond - 1956 Mercury Mark 55E