That is the issue we will be facing soon. The original (late model) Westail 42 came with about 450 gallons. 100 fuel, 350 water. The latest tank models using real measurements give a total of about 470 gallons. That includes the necessary gaps between the floors and the floor supports.
One could reason, that, if 100 gallons of fuel was sufficient for the diesel engines back then, it should be sufficient now, what with the increased efficiency of engines. But, engines are more powerful.
Boats did not have water makers back then. We plan on having one on this boat. So, do we need 350 gallons of water?
We could cut down one or both of the center tanks and have a nice space for a large battery bank. Location of the batteries is still being determined. The W42s I have been on have seen them scattered about (under dinette, in hallway cupboard, engine room).
But, we dont have to decide on the tank/battery issue until they have to go in.