I like to mount the tank a bit above the carb on a gasser. Two reasons. One it will be easier to get a prime to the engine. Two, some gassers like to go a bit rich while inverted. When inverted the tank placement would then be below the carb making the pump work just a bit harder. That seems to lean the mixture just enough so that when properly tuned there is no noticeable fluctuations in rpm between upright and inverted.