As long as the engine is above the fuel tank you shouldn't have any starting/ idiling problems other than over priming.

I've mounted O.S. .40's upright, sideways and upsidedown and they all ran fantastic. Connecting a tube onto the muffler and running it to the trailing edge of the wing will help with keeping fuel to a min. When flying rc planes of any type, I push them well beyound what they are designed for and fuel gets everywhere, but if you fly somewhat normal you shouldn't have to much to wipe off, goodluck.