Several guys built the boat with stock fir lumber 1.5 inches. I think you are right tho, the planing back was to allow them to bend.
Are you building a Jumbo? If so, the turn up at the bow isnt much, but I cant see how you will get them to bend much. Id do a test first and see. Maybe at only 4 inches you could get them to bend "some"
The shelves lay in the molds, pretty much defining the shape of the hull. The molds are set at different widths and heights.
by the way, I used the 1 1/8 plywood this time. I was more happier with regular lumber but guys say the ply works. But I had some 3/8th cut offs from the sides that were glassed so I flipped these over, and glued them to the shelf (bottom) so my underside gunwales are already glassed. the book says to just paint a couple coats of epoxy on the underside of the gunwales