1975 SeaCraft CC , 18' ..Transom height change ??

[McGillicuddy]

Welcome aboard, striker.
That is correct, a 25" transom requires a motor with a 25" shaft. A 30" shaft would run way too deep creating drag and very inefficient propulsion. In fact it is quite likely that you would never attain optimum rpms putting your motor at risk. When you get around to mounting the motor you'll want to set it at height where the anti-cavitation plate is about 1 to 1-1/2" above the keel at level trim (anti-cav. plate parallel to keel).

If you currently have a good 20" shaft motor and not certain about buying a new one, there is likely a 5" shaft extension kit available for that motor for $500-600 from Bay Mfg.

Good luck with the project.