So they would come out without too much work just removing the plywood top, no joinery to deal with. I think they're the original 40 gallons SS tanks by the looks, and seem to be fine after 50 years. They're plumbed with copper water lines and flare fittings on the tanks, which makes me think they're original.

The fuel tank, 80 gallons, is built into the hull just above the lead ballast in the full glass keel. It has four large steel covers bolted with a hundred or more bolts to the glass flange. When that has problems, the usual fix is to abandon (leaks come through the keel bolts) the tanks and have new ones built to place inside the old tank area. You lose some capacity but can still end up with 40 gallons.

That too is easy to access but would be pretty costly simply to have the tanks built and of course replacing any tank isn't going to be done without some work involved. So far, no problems with the fuel tank either after 50 years.