I have some 'thoughts'. I have a pair of long, thin tanks that I 'thought' I would install either side of the engines, to hold another 60 gal of diesel. But I 'thought' wrong, I cannot figure out how to get them in there without either removing the engines or cutting out some floor supports. So, I am keeping air in them, in a garage, until I figure it out. Meanwhile, I am considering adding some diesel tankage under the galley sole, using a flexible tank. Both projects are 'way down the list, however....

My big problem is that the cockpit sole is all one piece, and does not come up in sections. Don't do this. Make sure that your whole cockpit sole can be removed for working underneath. Meanwhile, whenever I need access to something, I add another little round plastic hatch. My cockpit is starting to look like a lily pond.

If you install a long narrow tank, do not put it athwartships. When it is half full, the fuel will follow the list, making the boat lean more. It will drive you nuts trying to keep the tabs adjusted. Better to mount a long tank fore-and-aft.

I think that if I had a proper hatch in the cockpit, I would be able to slide a skinny tank in and push it forward into position. But as I just pointed out, I have no hatch in my cockpit at all. The previous owner wanted it to be really, really, really watertight.

I don't know if you can use flexible tanks for gas, but it would take extra care, given the nature of gasoline.

I have a starboard list...but then that's the side of my helm and I have to go on a diet.
What side is your helm on Jeff?