Re: additional potable water tank location
I do think 400 lbs right over the rudder stock will have a major effect on fore/aft balance. When the PO added two extra batteries to the lazarettes, that raised the bow about 2 inches. Now if I don't keep the bow water tank full, the boat looks too high at the bow.
Also you want to pay attention to placing the water tank as low as possible so as not to raise the boat's centre of mass too much, effecting the righting moment. In a knockdown, it could make a real difference to recovery.
Where are the existing water tanks?
How about adding qty 10 of 5 gallon jerry cans? They can be placed around the boat, nice and low, and placed so as to not upset balance (or even to improve it).