It's real hard to evaluate without knowing the boat. Usually where there is a will there is a way.
Are you sure you want more fuel
tankage? Many boats sit with too much fuel tankage and the fuel gets old and poor. Some people consider less tankage and therefore have fresh fuel. Just a thought.
Is cutting the tanks apart in place an option? You can buy a Bosch nibbler to help with the cutting. Another option is the cut the top off the tanks and put smaller tanks inside the old ones.
Often boat furniture is built as removeable, although it's not obvious...but often not too.
sold with minimal openings can be carefully cut in ways to make the whole sole a patchwork of removeable hatches.
Cut the openings, reinforces the openings, recut and trim the mating edges of the hatch
and the opening.