On my 37cc, the
fuel tank is under the salon sole. There is both a water tank and a
fuel tank there. On mine,
fuel tank to starboard, water tank to port. There is a small access plate in the front of the salon, about centerline. Doesn't give you much access for doing anything though.
One consideration of adding a larger tank that is not low, and close to the centerline of the boat is the weight of the tank with
fuel. If put over on one of the sides, you will probably introduce a heel of the boat on that side.