I have a 89 s/c Starcraft with a 70Hp Yamaha. It has an 18 gal. tank. I know because I wanted to replace the tank and ripped the floor out. You don't want to do that. Left the tank in after I drained it and replaced the floor. I think your boat is seriously overpowered with a light weight aluminum hull. Forget about the mpg's. It would probably run for a few trips without refueling. I always top off the tank when i take it out from the ramp. Thats the problem. The "tin" boats don't use that much fuel and there is always some crud and water leftover. Drain the tank at the end of the year, refill it, add the stabilizer and have a good trip in the spring.