Thanks for all the suggestions, people: bladders were suggested to me by a boat repair guy, and they were but a passing fancy.
Plastic tanks of the HDPE variety might be nice and I concur that four 50 gallon tanks make considerably more sense. There's no reason I can't have inspection ports and the stand-offs, bracing and strapping are simpler with a smaller tank as long as I eliminate any chance of chafing or friction.
I would do a fibreglass keel tank, but these are not keel tanks, but "either side of the engine bay" tanks.
I'll throw out another question: What do people think of the new(ish) snap-together plumbing fittings for marine use, given the low pressures involved and the tendency of hoses to discolour and get grotty?
Here's an example of what I mean: it strikes me that from deck fill to tap, the less metal I have the better, which is the opposite of how I feel about through-hulls and fuel manifolds.