I just finished this project and I thought I'd share my process and things I learned for the next person who has to do this on a similar boat. I mostly used the instructable at https://www.instructables.com/id/Transom-replament/?ALLSTEPS as a guide.
The first step was to remove the two corner brackets and all 36 rivets. The biggest challenge here was starting a straight hole in the center of the domed rivet head, but good center punching made it not so bad.
As Southern Appal predicted, I had to bend the lip up to get the transom board out. I spent a long time figuring out how to do this. I tried getting a piece of steel under there and bending the whole thing, but what eventually worked was these special vice grips (thanks to my neighbor).
I ended up having to bend up the first angle (the tab of metal pointing down when I began) and then bending up the resulting 90 degree angle section by section until the sheet metal was all one plane. I worried a good bit about bending the whole transom out of shape, as it started to bow considerably (with the center protruding like in a convex shape). I ended up C-clamping a 2x4 and piece of angle iron across the back. Not sure if that was necessary, as the whole piece of sheet metal actually became concave after I took out the board.
The board came out without too much trouble, just started prying with screwdrivers, and then it came out.
And here's the obligatory, gratuitous shot of the completely rotted out transom board, which was ready to fall apart and had a thriving colony of small insects living in it.
My board needed to be about 1 7/16 in. I ended up getting nominally 1/2 in. oak plywood and sandwiching three layers together with gorilla clue, per
the instructable. For some reason he used pressure-treated plywood, which, as everyone on here says, is a no-no (or perhaps it doesn't matter due to the fiberglassing coming up next, in which case pressure-treated is unnecessary anyway). So I just used high-quality, non-treated oak plywood, I think seven layers per sheet.
By the way, the Gorilla Glue cleanup is much easier if you just chisel off the extra bubbly bits after it dries. Don't bother trying to clean it with a wet towel when wet as the bottle says, the stuff sticks to your fingers and is a pain to get off.
The issue I next worried about was totally not addressed in the instructable, which is the added thickness of coating the board with fiberglass jelly. I was also replacing a piece of masonite that was the motor scuff plate, which I made out of 1/4 in. oak plywood which I was going to fiberglass. Since the thickness of that piece was not crucial, I fiberglassed it first and found out that the fiberglass jelly added about .100 in.
Since the plywood layers I had glued fitted exactly into the channel at the bottom, I planed the board .100 to accommodate the added thickness from the fiberglass jelly.
I found out that the jelly was much easier to apply smoothly if you put in less hardener than the can recommended (the instructable talked about a pea-sized amount instead of a 3-in. strip). But that does change the curing time from under an hour to several hours. The first layer went on pretty rough, then I did a second layer just to fill in the low spots. An orbital sander pretty quickly took care of the high spots and rough edges. You really need a respirator for both applying the jelly and sanding it. That stuff will make you woozy in short order.
Miraculously, the board fit in quite well. I had a helper stand to the side to check squareness while I drilled the holes through the transom. Everything lined up well enough.
The next challenge was getting that darn lip back down. This turned out to be a four-step process, using the vice grips, then C clamps anchored onto the bottom channel,
then hammering with a rubber dead-blow hammer, then C clamps again, applied horizontally this time, to bring the upside-down U back into shape.
Next I replaced the two corner brackets, using pop rivets. I had to drill some clearance behind the ones that were directly above the new board.