Overly increasing the capacity of the main cockpit fuel tank will have the adverse affect of changing the weight distribution of the boat if the total capacity of onboard fuel is accomplished only with this tank. While it is possible to obtain as much as 350 gallons of capacity within the single cockpit tank, this would add as much as 800 lbs of weight to the aft section of the boat, resulting in the boat not resting properly in the water & a poor running trim without using excessive trim tab adjustment.

Excessive trim tab adjustment would also adversely affect speed potential and fuel economy. It is my recommendation that if more than 200 gallons of fuel is desired, that auxiliary fuel tanks be added. Optional areas for auxiliary fuel tankage is the center bilge area between the engines and below the cabin sole. The main tank should be restricted to a maximum of 200 gallons, which would allow a fast cruise range of around 250 nm when combined with the Yanmar 6LY diesel engine installation.

If two auxiliry tanks were imployed, the fuel capacity could be enlarged to around 280 gallons, increasing the cruise range to around 390 nm. However, I feel that this much range doesn't warrant the loss of space that could be needed for other systems and plumbing access. A single 40 gallon tank, either below the engine companionway or below the cabin deck, would yield 240 gallons and a range of well over 300 nm...

The two images below show a possible configuration for your onboard tankage, including fresh water and holding...

I would further recommend that all tankage be configured and installed prior to installation of bulkheads and compartmentation.