I've done a couple and will offer a few, but Charlie will be along soon and he'll have more.
Stay with the original outline, use a batten of 1/4" by 1&1/2" x 8 or 10 ft. to trace an outline on marine ply 3/4".
Epoxy paint or oil based paint on the undersides; rough it up with 60/80 grit sandpaper first.
If you have to put it in in sections because of shape or other reasons make them fit as well as possible before setting them in. You could use a marine adhesive on the stringers when you're about to screw them in. Stainless screws and BE CAREFUL when you get to the outer edges or you'll send a screw through the hull. Use shorter screws as you go out! You only need to set the board to the stringers, not the hull! (yeah I did it once)
You'll want someone with fiberglassing experience to set the sides to the hull, but with no one available, pick up a copy of wes' marine's book "fiberglass boat repair". It'll show you how to set tabs and form the glass correctly. It's an inexpensive book and well worth getting. You want to duplicate the original construction if possible and if that means forming a "ledge" to place the floor pieces on, do so. Depends on the way you'll use the boat. If you're hauling in crab pots over the side and you're 225 lbs., you may want to do that. But if you're using it as a weekend runabout and won't be putting lots of pressure on the edges, and assuming you've got a stringer system that runs to the outer edges with supports, you may just screw it down and glass the edges to the insides of the hull.
Glass the new floor after sanding it to rough it up and I'd recommend using the more expensive but much stronger laminating resin first (on all critcal areas) then brush on a coat of sanding/finish resin.
It's not rocket science. Take your time and think ahead of the steps you're about to make. Good luck, it's actually fun, especially when you get to shop for new seats and stuff, like a swivel helm seat!!
Buzz
