I'm helping a friend replace a s/s under sole water tank with 2 new
ones. The old tank was held in place with 2 pieces of 2x4 that wedged
the tank against one bulkhead on the aft end and with the 2 pieces on
2x4 wedged into another bulkhead on the fwd end- so the tank was held
in place longitudinally by only the end of the 2x4 - with the 2x4
positioned on the "hard" top edge of the tank where the welded edge
was. Eventually the welds cracked from the concentrated pressure of
the 2x4, and the tank leaked.

So how to hold the new ones - it's about 9" to the bulkhead in front.
The tanks lie about 3" above the FRP hull.