Fuel Tanks
I am in the process of rebuilding the rear cockpit on 360 Commander (it was rotting on one side and sinking onto fuel tanks); the fuel tanks were pulled last week; I am very fortunate to have a friend that works on tanks for a living and is going to test them and/or refabricate them to my choosing.
My boat has 2 tanks that are 200 gallons each; after studying the rear cockpit area I have contemplated having him make a single rectangular tank that would be fitted over the center line of the boat; it would essentially rest on two stingers and some horizonal wooden supports; my reason for taking this approach is that it will allow for easy access to the hardware for the shafts if ever needed (previously under the tanks) better access to the couplings for shaft packing; it will also permit the installation of two vertical supports for the rear of the salon floor; with this option I will see a loss in fuel capacity down to about 300 gal from 400gal.
While I understand what you are trying to accomplish with the single tank , I'd opt to stick with brand new aluminum twin tanks. You could probably adjust the footprint a bit to allow improved shaft log access. Your friend could probably fabricate some stainless supports to give addl cockpit sole support.
I really like the ability to bypass one tank if needed and run off the other.
Having good access to your centerline bilge is quite nice too as you are able to clean it and access your midship bilge pump pickup.
Here's the tanks in my 35 . I did go from roud tanks to rectangular ones which gave me increased fuel capacity while also gaining the ability to crawl between them to get to the lazerette without having climb out from engine compartment