That particular tank setup actually sounds fine. A lot of Great Planes and/or Thunder Tiger fuel tanks come set up this way.

The more likely issue is the level of the tank stopper relative to the needle valve on the Evolution .40 engine. The needle valve needs to be at least as high as the centerline of the fuel tank or the engine will tend to syphon fuel and flood while it's idling.

You can play around with adjusting the tank height relative to the engine placement, but simply mounting the engine upright or at 90 degrees instead of inverted will most likely be the quickest way to happy flying.