You should be experimenting some sort of small splash out transom, bear in mind that water flow is hitting the round portion of lower leg at speed and not the edge portion where water flow cuts as cutting a cheese.
With mate sitting at bow, with perpendicular engine sitin on transom go for a wot spin, when on plane pull your head out transom and check water flow passing through lower tail, if back splash out is minimum leave it there.
Doel fin must ride paralell to water level when sib is on plane, if not, will create extra water drag there. Besides a pain in the but when placing, retrieving engine from transom, could turn into real garbagge collector, collecting kelp, plastic bags, banana peels, you name it when boat is on it's way.
If wanting engine/transom height boating perfection match make a SS 5" engine mount..