Depending on the clearance above the tank, 6 inches should be plenty to get in there with a brush or similar implement and whatever cleaner is appropriate (after draining the fuel, of course). One or more baffles may exist in the tank, so be sure you get to everywhere that you can. I can't advise on what cleaner to use, but you shouldn't have any trouble finding threads on that topic. Rinse appropriately and allow to dry. Then replace the fuel, after polishing if you so choose.
I've seen tanks cleaned with everything from a bottle brush to a 36" wooden dowel with rags rubber-banded to the end. That one was a deep 100-gallon tank with two baffles, and three access panels, with plenty of clearance above, thank goodness. Improvise if you have to, but it is important to physically remove as much crap off the surfaces of the tank as possible.
