Gasoline or alcohol ? You are probably okay either way as we ran less water with more HP and on gasoline. Do you plan to run a thermostat? Alcohol does not require very much cooling water, in fact it is difficult to preheat the engine with alcohol. The original tank that I had made was about 16 gallon. It cooled the engine with no problems but now we are making more power and more heat. I built a new 30 gallon tank this last year. Even though we will make even more power this year I do not plan to make a bigger tank.
John