Tank position is important. You may not have a lot of vertical adjustment in the mustang though. Your engine does not care weather it is upside down or not when running if the tank is in the right spot. If it is high your engine will go lean inverted. If it is low your engine will go rich inverted.
Also syphoning is an issue if the throttle barrel is closed. It is just important to be aware of this as freely flowing fuel to your engine is never a good thing. Again if the tank is positioned properly it will not syphon.