I would totally remove the stringers instead of trying to reuse the shell of the fiberglass. Do like Salty said and cut them with a sawzall blade as close to the hull as possible. If you're really nervous about getting them back in the right place, do one at a time. Cut it out, grind, clean, cut, shape, fit and bond in place, glass, then move to the next one. You have the other stringers there as reference points to get a level surface for the deck and to take measurements you might not have thought about.

I used resin sanding discs rather than flapper discs. I primarily used 24 grit IIRC.