RE: Gas engine tank position, does it matter much?
Put the tank on the CG of the airframe and don't worry about height.
Rather than use a refueling valve, try a Fuel Dot type filler. Use a Tee in the carb line that has a hose to the outside world for refueling. But keep the refueling point above the tank so it doesn't drain out while you have the "cork" removed.