The jack plate will allow you to raise or lower the motor so that the cavitation plate will be at the proper level. I misunderstood what you were trying to do. You have a short shaft motor with a 20" transom? You will probably need to find a longshaft motor since you only have two knee braces...
If you can afford it get a manual jack plate to raise the engine.....If I had to do it all over agian, I would have found a long shaft motor for mine. My boat runs fine, but the old yammy longshaft was a bit faster. Next boat will be bigger....probably a 16 or better.
If you want to be blown away check out this build.Project "A Little Snookered" viewtopic.php?f=21&t=22132, I got a bunch of ideas from this build....one of the best on here if you ask me. There are many mirrorcraft builds to choose from. I like the way mine turned out, however through many...