US Coast Guard HIGHLY recommends that ALL portable tanks be kept outside of the watertight interior of boats. Yes, you could keep a portable tank below deck (not sure what your insurance company would think... but?) however, if so, it better be strapped down and have at least a 3" diameter vent hose leading from below the tank to a cowl facing aft above decks to exhaust fumes as well as a minimum 3" diameter vent hose leading from about the level of the tank fill cap to a forward facing vent cowl on deck to vent air in. Adding an exhaust blower to exhaust the fumes out the vent would be better (use a USCG Approved model) and that blower should be run at all times when any possiblity of a spark exists inside the boat. Now you would also need to be sure to check for leaks in the tank and fuel hose every time you go out (wouldn't want ANY gas leaks inside the boat!!). Finally, you should replace your fuel line with hose approved for inboard engines, and connect direct to tank fitting with no quick-disconnect.
A permanent tank is considered safe below decks because; a) tank was designed and built to be below decks, b) tank is vented overboard, c) tank is fastened into it's location to prevent movement d) tank is filled from an above deck filler that is located to direct any spills overboard, e) all hoses are USCG type A-1 that are approved for below deck gasoline without leaks or vapor losses.
If it were my boat, I'd either build a fuel compartment that is vented directly overboard and completely separated from the interior of the boat, or I'd keep the tank chocked in the cockpit footwell (about an inch or so above sole to allow air and water to flow under) and construct a lid to cover it to keep the sun and weather off the tank. On hte 25 I belive that would place the tank right over the scupper allowing any fuel leaks to drain overboard and there should be plenty of airflow to dissipate fumes.
We used to store the 3-gallon gas tank for our CAL 21 under the cockpit, we had vents for the area and the tank was strapped to a platform. One time as I was removing the tank to fill it one of the hose connections came apart and we spilled less than a cup of gas into the bilge...... it took a few hours to fully dissipate the fumes from the bilge, during which time we did not want to move the boat or even remain aboard. The next season (incident happened as we prepared for haulout) we built the gas tank locker shown in my earlier posting.
About 10 years later on haulout day as I turned the outboard to back around the floating dock to get ready for hauout the battery charging wire from motor to battery got caught on something and it grounded on the engine causing a short that melted all the insulation off the below decks part of the wires....... we were able to unhook the wires before the fire spread beyond the insulation of the wires.... but imagine if the gas tank was still stored under the cockpit (about 18" from that burning wire)!