If you have at least 5 or so of clearance behind the tank you can use a lag bolt and a socket wrench to turn the bolt. Of course you gonna need the shortest bolt you can find because of the space. Place a fry bar or a wood, preferably 1X between the wrench and the wall and put pressure against the wrench. Now start screwing the bolt, it may only take a few turn or so to get the bolt going. Then you just take the fry bar out of the way.
This is exactly what I did when I forgot to drill a pilot hole for the hanger when I built the stand. Have fun