If you're mounting your engine inverted the carb will be lower then mounting it horizontal...I have my OS 55 with a pitts mounted at 90* and the center of the tank is in line. I also filled the top inside of nose with light foam padding to cushin the tank. This also creates a tight fit and holds it in place.

I had two Venuse 40's from Great Planes. I originally installed an OS 46 FXI inverted and it was a nightmare. Tough to start unless you turned the plane upside down, even with an idle-bar glow plug and it went deadstick about 70% of the time when you throttled back to land. After I turned the 46 to 90* with a pitts it was perfect.