2 years ago, I done my 18' Sylvan. I went a little further, in that I modified beyond just replacing. Besides the floor/foam, I removed the splashwell, replaced the transom wood with Coosa Board Bluewater 26 and riveted an extra sheet of aluminum skin to the outside of the transom in order to raise it to about 21 inches. Then, I used aluminum from the splashwell to build a motorwell. I put a 28 gallon gas tank in the floor. Then, I built a rear casting deck, with storage underneath, where the splashwell was. Now, I can use the whole boat. It's like my boat got 2 feet longer as far as fishability is concerned.
My boat is a center console. It had 2 floor mounted pedestal seats behind the center seat. That was where it appeared the most water got into the foam. So, during the re-build, I left them out. Now, passengers set on the rear deck, and use the Bimini top as a back rest.
I'm here to tell ya, about half way through, I started to wish I'd never started, but, after I finished, I'm convinced this is how it should have came from the factory. The gas tank in the floor has been one of the best things. Not only did that free up a lot of space, it also lowered and moved forward the overall center of gravity.
Also, I now a lot of people will say don't bother with the expanding foam, and just use pool noodles or whatever. I believe the foam, while not solely there for structure support, it does provide some. Here's why I say this. As you can see in your boat, the factory riveted everything together, including the floor. Then, if it's like mine, they cut 8 holes in the floor and injected the expanding foam. In my boat, they put to much in. It caused a slight crown in my floor, but, more than that, it busted the rivets that connected the stringers to the ribs. There were over 50 busted rivets. The stringers were only attached at the first rib and the last 2. Every rivet in between was busted by the foam. The force from expansion has caused a 3/8" gap between the stringer and the ribs. Now, IMO, had that foam not been so rigid, this would have shown up earlier. As it was, it remained solid from 1986 to 2011, even though the stringers weren't attached. If I hadn't torn into it, I still wouldn't know.
You probably already know, but the iboat forums are a great resource. Also, McMaster-Carr was the best source I found for rivets and screws. If you do order from them, it will usually be delivered the next day. I found that to be awesome, especially if I'd forgot something, I'd call them up and have it the next day.