RE: REMOTE FUEL FILL VALVE
As you noted the connections on all such fill valves are inserted in the clunk line from the tank to the carburator or the main remote needle valve.
Overflow is the same as every simple two line system and will come out of the vent line from the tank Which is plugged into the muffler which is normally unplugged at the muffler to watch for a full tank.
My personal reccomendation is to avoid any third line as it is actually redundant and unnesesary. Also I would even avoid even the refueling valve altogether as they will at some point get dirt into the valve and cause running problems that will cause much headache. Its far better to use a simple two line system system and it even works well in cowled engines when done right plus it provides an exposed loop for you to use when learning the pinch tuning technique.
John