I just put a new tank in the front under the goucho of a 1963
Measuring the dimensions will give you the most capacity. The tank in the Globe is close to 30gal, and was purchased from Ronco Plastics in Tustin CA. It is listed in their Marine tank section, for some reason. $ 240.00 or so...complete with all fittings.
Measure the width of the area the tank will reside in, then from the front wall to teh begining of the rear drawer section. Then, measure up to the bottom of the slide rails for the pullout mechanism. That's your basic dimension. I would subtract a half inch or so, maybe even a full inch, for wiggle room.
The new tank is made from polyethylene, and is rectangular. You might have a round galvanized tank under there now, or maybe a poly tank, take a look....