Transom height

Picked up a older starcraft and knew the transom was shot. Bascically I am stripping the boat and using it for duck hunting small lake fishing.So cosmetics is at the bottom of the list. Now has a 21'' transom and I will have to lower it to 15'' to accomodate my 57 18 hp Johnson, rating is currently 65.Should I make the height 15'' or raise it to where the cavitation plate is just below the waterline to reduce drag, which we did as kids on the hydroplanes we ran.Any suggestions on woods to use, [glue plywood pieces together,then expoxy coating was my first thought] or some other material.

i would set the engine height so that the cavitation plate is level with the bottom of the boat.