Carl,
Not sure how much power you plan to use, but the WC's are rated for less and have a little less freeboard. The s-14 is a great boat and very stable.
As for the bench seats, no, I'm not putting the middle bench back into the boat. But I will be supporting the structure of the boat with side rod boxes and storage that will span from the front bench seat to the transom of the boat. After I put the floor in, the side storage boxes will be secured to floor and to the side. This should maintain the lateral structural integrity of the boat and help prevent twist. When I get to that point, you'll see what I am talking about.
One thing, if you plan to notch the middle bench seats, I wouldn't cut all the way down to the floor. The bench seats also act as ribs in the s-14 unlike some boats where they just provide lateral support. If you see the SSV's, you'll see that they leave a few inches from the floor in the walk through benches. On my boat, the aluminum angle I used as crossbars reinforce the bench supports to provide for the lateral support.
Not sure if you saw this thread, but he has an s-16 which he made into a walk through (https://www.tinboats.net/forum/viewtopic.php?f=21&t=15212&start=0).
Ictalurus,
For flotation, I will need to figure that out as well. The volume under the floor will not provide as much as I'd like.