If your question is whether to remove the tank, absolutely. First and most important, you can evaluate the parts of the fuel tank you can't see, especially if it's foamed in.
The plywood could be mush.
You can get at the stringers. We drilled some exploritory holes low on the stringer sides and sure enough, we vacuumed/drained out some water. It was nice to paint the whole thing before installing the new tank.
Regards,
Otto