your right about the metalbracket, it would not work on a closed transom. the armstrong in the pic with the motor or the first pic would work.
here's a couple of links to help you:
http://www.wellcraftv20.com/communit...version&page=3
http://www.wellcraftv20.com/communit...2Fo+conversion
i do think on an 18, squatting in the stern is going to be a problem. you will probably need to go to an older 4 cyl 2 stroke & move as much weight forward as possible.