I feel your pain, as I've been there on multiple boats in my past. And I'm VERY hesitant to chime in here, as this subject is (obviously) VERY IMPORTANT, very complicated to get right, and not typically a DIY job. Having said that, here's some suggestions:
a. Coastline Equipment in Bellingham (https://coastlinemarinetanks.com/
) are your friends. They are most assuredly the premier fuel tank manufacturer in the PNW. They have oceans of experience in not only the fabrication, but the design of tankage as well.
b. 316L is fine material for water tanks. But 5000-series fuel tanks will cost ~1/4 as much, last just as long (assuming they're properly installed), and will weigh 1/2 as much.
c. Pay attention to the method of plumbing your tankage pairs. Your sketch shows what looks like a simple pipe nipple between the two tanks. Don't forget you can't make up a pipe nipple with the tanks down in the engine room! You would have to rotate one tank or the other 360 deg. to make that connection. Your only option, should you desire to mate the tank pairs end-to-end is a pipe union. And, unless you want to add multiple deck fills and vents, your union(s) should be sized AT LEAST as big as the deck fill diameter. So...that's something like a 1 1/2 to 2 in diameter union. Which will require something in excess of 8 inches between the tanks to install and make up. This will adversely affect your available tank volumes.
d. 6"-8" Side inspection plates are highly recommended, on EACH tank, in way of EACH baffled area.
e. Don't forget a vent of each tank is necessary.
f. If you already have two tank fill ports on deck, and two vents, then each tank pair can be combined for both fill and vent.
g. I see little need for a crossover port-to-starboard. You can add a crossover on the fuel drain manifold, if you really feel the need. But below the tank tops (IMHO) is simply not necessary.
h. Sumps on each tank are nice to have, although difficult to accomplish in practice. Ideally, the fuel drains should be in the sumps, but again, tough to do in practice for ganged-together fuel tanks.
i. Another vote for sight gauges.
j. Installation of the new tanks is everything. And the devil's in the details.
k. Steve D'Antonio (https://stevedmarineconsulting.com/
) has been down this road about a gillion times. Suggest you chat him up, and consider engaging him as a consultant.
Design of a new fuel system for a cruising powerboat is a complicated subject, and is very much boat-specific. I would highly suggest you do your due diligence in searching out a boatyard and/or fuel tank design and installation company that has actually done this work, preferably on an identical boat, and can put their boots (and eyes) on the ground in YOUR bilge. Attempting this yourself via the WWW is certainly possible, but not easy, and not for the faint of heart.