The bottom of the hull and transom are cored--some with balsa and some with foam. The cockpit deck and cabin floor are cored and are part of the bridge deck structurally. There has to be a "beam" holding the two hulls and bridge deck together. The aft cabin bulkhead helps with this, as does the forward bulkhead between the area under the forward berth and the hulls on each side. The water tank can be on either port of starboard, forward of the fuel tanks. Some boats had a second water tank put in. The "fish boxes also take up area of the hull aft of the fuel tanks.
To put in extra fuel tanks, you would probably have to cut up the cockpit deck, eliminate the fish box (probably not a bad idea anyway--they all leak). The 150 gallons give you a good 200 mile range. You don't want to put too much more weight aft. The downfall of the Tom Cat 255 is limited tunnel clearance--it is gone at rest--and you need to keep it open when on a plane. If not you are pushing a lot of water...
To put in extra fuel tanks, you would probably have to cut up the cockpit deck, eliminate the fish box (probably not a bad idea anyway--they all leak). The 150 gallons give you a good 200 mile range. You don't want to put too much more weight aft. The downfall of the Tom Cat 255 is limited tunnel clearance--it is gone at rest--and you need to keep it open when on a plane. If not you are pushing a lot of water...