Is there any reason why I cannot mount the fuel tank sideways - that is perpendicular - at 90 degrees from fore/aft.
Hmmm? You should have the tank connections at or slightly above the carb inlet fitting, but side-to-side? Do you do rapid rolls? When you are rolling away from the side with the stopper you'll likely be starving the klunk. I don't know if it would be a problem in practice or not.