Re: Transom 15" to 20"
I'm presently raising the transom 3" on my 22' Grady White, but it is just one part of a complete transom rebuild. A ton of work.
I think for your application, without seeing any pics or having more knowledge of your transom layout, you may be better off getting an adjustable jackplate, or similar bracket that will attach to existing transom. You might be better off locating a shorter mid section for your outboard and switching it out. JMO, Mike.