RE: Fuel Tank Location
I personally like to mount my tanks on the same level as the engine. But have mounted many tanks below the motor plate in the past with great results without a pump. The motors used were OS. Other motor performance may vary.
I have even seen an OS46FX with the tank located in the hull, 6" below the motor with about 18" of fuel line, and with no pump. The motor didn't seem to care. The only thing we noticed was that it took just a short moment to prime the lines. Once the lines were primed, it ran fine. Like I said, different motors may act differently.
You could always bench mount the motor and try the tank at different elevations to see how your motor is going to react.