Hmm, you may have to get creative, with what is known as a universal joint for the socket in order to reach the bolt. Can you even see the bolt, or put a finger on it?
Also, be careful dropping the tank, even an empty 50 gallon steel tank is prety heavy, and fuel weighs almost 8 pounds per gallon, so a quarter tank would weigh an additional 96 or so pounds. If you have a floor jack, it is very handy to place under the center of the tank, both to lower, and to raise back up. You probably already know that, but the next guy reading this thread may not realize it until the tank is lying on his chest.