I think tank position is important in fuel delivery but I have been centered, below and above the needle valve. If your within in inch of center I feel your OK. The further one gets from center makes performance differ when manuvering. The difference in fuel delivery from full to empty should be about the same regardless of tank postion, since the change is only related to level change. However if you are 1" above centerline with your tank on the ground, then go inverted, now your 2" below center.

I do not like upright engine mounts because it is not attractive. I generally go side mount, good performance needle valve is accessible and hard to flood. Plus it looks better.