Engines do not really have an up or down side. There is no real right-way-up. The right way up is the way that fit's mosrt conveniently on your plane.

TThe only real recommendation is that the center of the fuel tank should line up as close as possible with the carb of the engine. Even this though is not really necessary any more. There is one concern with the pressure line from the muffler of the engine. If this is installed below the fuel line in the tank, then there is the possibility of syphoning when the engine is not running.

Bottom line: install the engine as is most convenient. Install the tank in-line with the carb, and make sure that the pressure line from the muffler is as high up as possible on the tank. It is often convenient with the pressure line to install it in such a way that turning the plane upside-down allows you to pump out all the fuel when the day's flying is over.

gus