ORIGINAL: downunder
It seems that Ultimate 40S is designed for an inverted engine so presumably the tank is set in the right position (it's worthwhile checking though). The engine doesn't care which way up it is once it's running but you have to learn a slightly different way of priming it because you're trying to get fuel to run up hill from the carb and into the engine. Also, inverted engines need a bit more care in setting the idle mix to avoid any stumbling after prolonged idling. Properly set up you don't need pumps/regulators or any other complications.
Yup, nothing is needed.
The Ultimate sits "nose up" when idling, normally, as it is a tail dragger, so with the tank inverted the levels are fine.
However I haven't had any need of doing this, nor is priming problematic. It just takes a little longer to start with an electric starter.... a bit more cranking to get the fuel up before the engine kicks in.
With the needles properly set the fuel should NOT siphon back out that quickly at idle speeds.
If you open up the throttle it will siphon back though.