From the sketchy details, and some even sketchier math, I'm guessing your pontoons are about 36 feet long and about 4 to 5 feet in diameter. If they are much smaller in diameter, you'll certainly not be able to displace 18 tons -- but you could still have a nice raft.

Basically you need a frame that holds your two pontoons apart, and parallel to each other. The construction is very much like that for a floor in a house, or a backyard deck.

Assuming you want a boat that is about 12 feet wide, you'll start by welding brackets onto your pontoons. Put these 16 inches apart, and spaced the same on both pontoons. To these brackets you'll bolt 12-foot-long 2x10 or 2x12 boards that go between each pontoon.. That establishes the width of the boat. If you want a narrower boat you can go with 10' or 8 foot long boards. If you have metal beams and you can weld these to the pontoons, then go for it.

If you aren't up to welding, you can create a rectangular box of 2x12s which fits over the top of each pontoon, and use metal straps under the pontoons to hold them in place to these boxes. Use wood framing to connect the boxes and lay a deck on top of that.

All construction of the frame for the deck should be held together with bolts and nuts if you can. You may need to use metal straps, or angle iron with holes drilled in it to allow you to connect these materials. Use lagbolts or big wood screws if you must. Nails will pull out. You can use them for holding the deck boards or plywood onto the 2x12 frame, but that's about it.

You'll need to put some diagonal bracing underneath these, and some plywood or common boards on top to complete the deck. You can make walls much like those on a shed, by building them from 2x4 framing, which can be sheathed on the outside with plywood, or siding.

To get this to move, you'll need a motor. You can use outboard motors, or you can use a modified auto engine. Volvo makes a marine drive unit that bolts onto their motors, and there are adapters available for this or similar drive units that will allow the use of many American V-8s.

I think the simplest way to get going would be to get two outboards motors and mount one to the end of each pontoon. You could steer by turning them, and/or by adjusting the thrust, and the power would be mostly in line with the drag of the pontoons.

If you are looking for something a bit more refined, Clarkcraft has plans for a 30 to 40 foot houseboat which is a bit more comprehensive. Try this URL

http://clarkcraft.com/cgi-local/shop.pl?type=item&categ=010&item=933302528&cart_id=aa05f181e2a215fa710109c50b7b39e0

Or just go to www.clarkcraft.com and click on the buttons until you come to houseboat plans.

here is a link to a picture of one of their plans.
http://www.clarkcraft.com/images/b317.jpg

Glen-L (www.glen-l.com) has plans for building houseboats on pontoons, the picture below is of their "Super Huck" design, with the optional cabin. They have a smaller "Huck Finn" version, and even larger models. Glen-L also has books on how to deal with motor drive units for boats, including a book on how to modify a shortblock Chevy V-8.

Check out both sites and look through all their online plans and you can get some ideas about how the cabins are laid out.

Kinda rough details here, but the last raft I worked on was in the last century, and the memory is fading.

By the way, you're going to have a heck of a time moving this thing once it is built! While the pontoons should support the deck, the deck may not support the pontoons should you try to lift the thing from the water. Trailering a 12 foot wide boat may not even be legal without certain licenses. So, think of this as a boat you'll mostly assemble on the beach, or in the water -- or plan on taking the thing apart so you can handle the deck as one unit, and each pontoon separately. That way you can put it away for the winter, or get it out of the water to occasionally paint it.

This is going to be heavy.

Hope this helps.

PGJ