The engineering term for the piece of plywood undernieth the tank is a "cantilever". There are design analysis methods for determining the proper size of the cantilever based on the weight supported, overhang distance and material used as the cantilevel. For a 6" overhang supporting a 55g I would guess at about a 1" piece of plywood should o the job with plenty of margin. You might never need it but I would sure want the extra security of supporting all 4 sides of a tank.
