Didn’t quite follow the Chevy ring gear approach but might consider--
Motor mounted pinion down on the upper, rotating column.
Drives into a twin flange bearing mounted shaft with shaft projecting down to the fixed column.
Shaft has Rex’s timing belt small pinion engaging belt around larger pulley fixed around the lower column.
Could use chain if belt is too spongy, or gears, but lube and corrosion problems.
Or, turn it all over, but may interfere with boom brace.