Quick and painless worst case fuel usage, divide the HP by 10 to get rough GPH use at full power. So figure 300 hp is going to use about 30 GPH at WOT. Getting into the more intensive math yields a value closer to 25 gph for that engine at WOT. Obviously cruising around at 3500 rpms will increase your mileage. IMHO, keep as large a tank as you can fit, if just going out cruising for a couple hours, you could always just fill it halfway. If using the original aluminum tank, take some time to insure it is leak free before going back in. I pressurized mine to 5lbs, then soaped all the joints and areas that had some corrosion, found three pinholes and got them corrected.