Hi Dave,
I restored a 1951 18' OT sponson sqaure stern not too long ago. It was for someone else and it's gone now, but I can tell you a bit from memory. I didn't measure the seat boards, but they were about 3/4" thick and made of fir or spruce. All 3 seats were mounted on cleats and only the center seat had brackets.
These brackets (see "before" photo) as well as the blocks at the junction of the inwales and transom and the one at the junction of the transom and keelson, were all attached by through-bolting with brass machine screws and square nuts. The seat blocks (two total, one on each side of the seat) were bolted at the top through the gunwale and at the bottom through the seat. Heads were countersunk, or course. machine screw a the gunwale was inserted from the outside of the boat through both outwale and inwale, and the nut was countersunk into the block, the excess shaft cut off, and then both the shaft and the nut were filed to be perfectly flush with the curve of the bracket. The bolt at the bottom (i.e., through the seat) was inserted from the top (head countersunk), and nut applied below the seat.
Can you picture this? I don't think I have detail photos.
Hope you're doing well,
Michael