if you don't care about having the tank on the cg for aerobatic purposes,(will require a pumping system) then put it as close to the engine as you can.

inverted engine mounting....
putting the tank high in relation to the carb can cause a siphoning effect into- and -down the carb... directly to the combustion chamber. the engine will still run good but will make the idling very finicky and sometimes impossible.

for inverted engines, it is a good idea to put the tank much lower than you would normally.