You can get a 14 gallon (18" long x 11.75" wide x 16.25" high) Moeller tank for $155 at Defender. A fuel gauge sender with a return fitting is an additional $42. If the 14 gallon tank is too high, they make a 12 gallon tank that is 20" L x 14" W x 11.75" high for about the same money. You could save about $30 by putting a T in the vent at the tank and connecting the return to it. After you get the main tank replaced, you can continue to use the Moeller poly tank as a day tank. Fill it by valving the return from the main tank to it so it will have only clean, filtered fuel in it.

Running the return to a separate Jerry jug will greatly reduce your run time since the amount of fuel circulated is much greater than the amount used unless you are running the engine up near top rpms. In other words, you will fill the Jerry jug pretty fast and have to stop while you transfer the fuel back to the main tank.

I installed a Moeller plastic tank on my power boat about 3 weeks ago. If you have a spot for it, it is easy to do