short shaft motor on long shaft boat

[ohenry]

Fresh to boating and due to a bad purchase and tring to make a bad situation work I need advice. I have a 16 ft (thin/light) fiberglass V-hull side control with a 30hp johnson (rope start short shaft unknown age but works well). The boat needs a 20" shaft motor (not 15" which is on the boat) the trasome is about 18.5"! I would be content if I could mount the motor low enough without cutting the trasome or off setting the rear weight distribution to much. I have considered getting a jack plate / reversing it to drop the motor but fear the possibility of not being able to tilt the motor or the motor being to low in the water hence ? h2o damage.
ohenry