Personally I've not run into issues with mounting my fuel tank above the carburetor of either my O.S. .91 4-stroke engine or my Saito .62 4-stroke. Currently in my Hangar 9 Piper Cub I'm running my Saito .62 at 90 degrees just fine (3 or 9 o'clock depending on your vantage point) and my tank sits 10mm+ above the carb. I've been running it that way for 3 years and never had any issues. As a mater of fact, it's my best running engine save for my O.S. .25SF.
Anyone else have any input on his question?