Welcome aboard! You should never say "all comments welcome" around here!
I'd frankly be more worried about the plywood core after 30 years. If the interior is wide open I think it would be relatively easy to design and fabricate appropriate bulkheads and glass them in place. Fibeglass is not going to take a permanent set, but it will easily flex or deform if not properly supported. Assuming the deck is on the boat and secured, I can't see missing the bulheads as being a significant structural issue.
That said. I am going to assume a couple of things I probably shouldn't and say: RUN FROM THIS PROJECT...it will cost you more in time, $$, and sweat than you will EVER be able to recoup.