It's one of the problems about ARFs. When you ASSEMBEL it your stuck with where they had room or just figured this is a good place for the fuel tank. Not sure how you have the engine mounted but it's one of those things you have to think about when ASSEMBLING. I have been able to get inside the tank area on some ARFs and remove the wood area the tank mounts onto and remove it to lower the tank. Other then that flipping the engine may be your only way.