Have you checked the under side of the hull under the stringers for gelcoat cracks, dead give away for soft stringers. Be really sure there is no no rot or your project will be like building a brick house on sand. FNQ did a great with his floor on the rebuild but of course it comes with extra $
For sanding fg and old gelcoat I have used the really course 7" sanding discs lowest grit size you can get. Cutting through fg the 5" 1.0mm cut off discs are the go.
I feel itchy just looking at fnq's. grinding fg is like hitting your thumb with a hammer good when you stop.
A mate put a home made pod on his 17' milligan after he took out a stern drive. made it out of ply and glassed it over, wasn't flush with the bottom but added good boyancy. looked a bit rough but did the job
Good luck with it
Wayne