The measurement you cite is indeed the one to which you match your motor's shaft. To adapt the 20" transom to a 25" motor you have a few options. I think you could research the costs of each fairly easily. <br /><br />You could change the 25" motor to 20". Involves getting rid of the extension piece and replacing the drive shaft and some other parts. Can be done, but I have no idea how expensive or difficult for DIY. <br /><br />You could get either a powered or manual jack plate. Easy to find cost (look at any major boating catalog, local shop or on-line). Easy DIY if you have a way to lift the engine. Would not only adapt the transom to the long shaft motor, but would allow you to fine tune engine height if you ever find that to be advantageous.<br /><br />Finally, you could have the transom built up. This is a fairly common modification on aluminum boats - don't know about wood/fiberglass though.
