Transom extension on a 74' 20' seacraft

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Hey guys,
first post for me, have found everyone's posts and replies inspiring and educational. I am trying to finish out a rebuild, which someone couldn't complete. He replaced the stringers, transom, and put in a brand new 50 gallon gas tank. I hope he did a good job. Right now, I have a hull which is starting to shape up on the inside, lots of filling and leveling. So far I have used epoxy fairing compound to join all the cuts together that this fella made to pull the boat apart, do his handiwork and then put back together---west system actually--which is probably taboo to most fiberglass veterans. But I am happy so far with the results---I think. Lots of sanding. But the transom is only 20" from the bottom of the keel. My research says i need to extend the transom up another 5" to allow for a newer style motor and so I don't risk large breakers coming over aft, or worse, taking on water while at dock---while no-one is there to notice. I think I could just add plywood to the top of the existing transom and lag bolt it down, w/ resin coated generously between each layer of plywood and then wrapping it all in fiberglass cloth, soaked in resin. The lags would take most of the weight, and the plywood would essentially become one uniform piece, right? The other option is to cut out a large swath of the transom, and try to sister on a couple sheets of marine grade ply, like guys have demonstrated on this site. Any thoughts? Also, is ACQ or pressure treated plywood the same thing as marine grade plywood?