[X(]The tank position had better not be critical on an engine made in this modern day. Now-a-days there are some hooligans who do outside loops [X(], flat spins [X(] , tumbles [X(], extended vertical climbs [X(], even hovering [X(] with their planes instead of just flying them in a dignified, upright manner [X(].
The tank and or lines should be arranged so that the engine has to work a little to draw fuel and so that you aren't bothered with syphoning. The purpose of bench running an engine is not to derive the final tune in the aircraft, regardless of which way the cylinder is pointed.
Prolonged idling will test your ability to set the low end, raw fuel will snuff out the plug if you are off by too much.