I found a replacement tank from Raratan. It is a 30 gallon unit. The model number is 30B134.
This view is a comparison between the old 25 gallon tank and the new 30 gallon unit from Raritan.
While I had the tank out I painted the void then I laid down a vinyl floor runner I got from Home Depot. I don't know it this will help any but it was cheep.
This view shows the new tank in place. And another showing the fittings, notice the hose clamps to reinforce the plastic.
I cut stringers out of pressure treated 1X6 yellow pine. I screwed them in place to support the lid and also for the tank Hold down blocks.
Her are a few photos of the completed settee.
Finally I placed the Y-Valve under the sink in the head.