RE: More Important?
If you do not want to mount the engine sideways (preferred choice is to mount it sideways), remove the cross bar over the tank and make a new hatch cover that is hollow (or curved using sheet balsa) and attach the cover by using magnets. Radio Shack has small, but quite powerful, rare earth magnets. Imbed the magnet into the fuselage side and imbed a small STEEL plate into the hatch.
I THINK it is Edmund's Scientific that has square magnets (about 60 of them for around $6 ). Each magnet is 1/4" square and has a 6 lb+ pull. I have one plane that uses 4 magnets on a 9" long 4" wide hatch (one in each corner). Doing maneuvers (Loops, rolls, spins, etc) at speeds over 90 mph at times, the hatch has never even shifted position.