Hey Striker,
Several of us during rebuilds have raised the original transom from 20" to 25" and from my personal experience I highly recommend doing it.
A 25" transom would ordinarily hold a 25" shaft engine. The exception might be if you wanted to use a raised jackplate mounted on the 25" transom which might allow a 30" shaft. I would not imagine the need for this setup though, especially for an 18 footer.
There are plenty of folks around here who are much more knowlegeable than myself and can provide great advice along the way. Posting some pics of your progess will help
tremendously!
There have also been numerous step by step threads posted on this subject over the years and I highly recommend a liberal use of the search function to see how other Seacraft owners have rebuilt their transoms.
Best of luck and welcome to the site
