It will be close. Your critical dimension will be the distance between the inner stringers. There are three pairs of stringers in that boat plus the keelson. The original tank deck is flush with or immediately on top of the first (inner) set of stringers. You can see them on either side of the tank deck in Bilgerat's pics.

The inner stringers are the forward extension of the engine beds on a single I/O. In order to fit a 15"+ tall tank in there, it will have to fit between the inner stringers. If the stringers are more than 29 5/8" apart, it may just work. You can probably measure between the engine beds in the engine compartment to see where you stand.
Dave

Strick and I both put fresh water tanks under the cabin sole. Mine is 30 gallons. Strick can chime in on his, but I'm happy with mine. With full tankage and some gear in the cabin, this is how mine sits in the water:

http://i655.photobucket.com/albums/u...ps76ecb74c.jpg

With the forebody lift this hull has,you don't want it tail heavy. Unless you like spending a lot of time in the air, that is. The landings are nice and soft. :)
Dave

Ahhhh.....life at Breezy Palms is so nice!!! :)

when I pulled the floor under the tanks there was a 3/4" strip of plywood screwed to the top of the center keelson , the 1/2" plywood floor sat on top of it and it was flush with the top of the stringers. heres a pic I took of the aft tank bulk head wall as I was glassing it up, you can see the way the center keekson is rounded on top, I raised it with glass and ground it flat to bring it up to within 3/4" the level of the stringers and used 3/4 marine ply for the floor.

So I dug around a bit in the bilge today, and the keel stringer is a *lot* lower than the tank bottom in this hull (it IS a deep vee). Sitting here now I realize I threw out the paper I was working on, but it seems like from the rear bulkhead to the forward tank bulkhead, it is 20" from the top of the keel stringer to the deck. Call it 19" clearance in the middle, enough height for a 16-1/2" tank in the middle 24" or so, and 12-13" outboard of that atop the stringers to 34" wide. Plus the 2-2-1/2" for a filler hose.

Does anyone know if the keelson is full of wood? I am thinking backup batteries for bilge pump and VHF down there. But I don't want to screw into a 2x6 or something.

Keelson`s eat wood until they are full. They can wipe out a well stocked pantry in no time.
The good news is you can laminate over and lose some wax. Quit chuckin` my wood.

Cheers,
GFS

keelson is wood covered with glass, likely to be rotted inside.

I was hoping for a different answer, but suspected as much.

I pulled tanks. I will be putting in plastic tanks. If anyone local has an interest in them, let me know.

edit: nevermind, they are taken.