If the engine is mounted inverted, it will siphon fuel into the carb regardless of the tank position. Plan on starting it inverted, and flip it over when running. This is a technique discovered a long time ago by the control-line flyers when using an inverted engine. You will have to set the needle on the rich side at the start of the run to compensate for the lean setting that results when the fuel level gets below halfway. 2 ways around this; one, you can use a fuel pump or a regulator. Second, you can use a gas engine that already has a pumper-carb. There used to be a glow fuel regulator made by a company called Cline. An internet search might prove fruitful.