I appreciate your quick feedback. I don't see how you could notch it and get the wood behind the side seat pods. I have to have enough wood to attach the side seat pods and the back handles of the boat as well. Based on some other post, it appears the right way to do it is below but I don't want to do all that rivet work if there is a way to get around it. How could I notch it and get the board in that bottom brace and behind the side seat pods?
First, remove the knee brace(s). Simply remove the bolts going through the transom, and drill out the rivets holding it to the hull.
Next, remove the side seat pods. Again, remove the rivets here - don't cut any sheet. It should have rivets through the side of the boat, and through the bottom of the hull.
Remove the L brackets in the corners (shown in your second picture). Again, just rivets and bolts.
Now, remove the strip on the bottom of the wood. It should be a Z shaped piece of formed sheet. Just as above, it's just riveted to the hull.
At this point, the wood should come right out - it is cut undersized enough to drop down an inch or so to clear the top cap. If it doesn't slide right out, figure out what bolt is still through the transom that you missed.
Install your new wood (or better yet, your new custom fabricated aluminum ) core, and replace everything in the reverse order.