Given the amount of work you've already put into this project adding a gear rack as an insert into one of the flat surfaces of the overarm and a spur gear and handle on the back side of the mill would seem to be a walk in the woods. That way you could easily move the overarm.
Or you could really go overboard and use your shaper to cut a gear rack right into one side of the overarm.
Or you could really go overboard and use your shaper to cut a gear rack right into one side of the overarm.