water over the transom on Diablo

I have found the only problem with my Diablo so far. At higher speeds she churns up water over the transom in the middle
where the motor bracket is. I have determined the cause to be
the long shaft motor.
Although the plans call for a long shaft, the 2001 25 Merc that I have sits about 4 inches too low in the water. Has anyone experienced anything similar and if so what was done to correct it?

 

Water over transom

I had the same problem with a 25hp long shaft Merc outboard on my Diablo boat. I epoxied in a built up plywood piece to raise the transom height and thus the motor. No problems now with water over the transom, even at top speed.
The outboard motor's cavitation plate and attached "Dolphin" fin are now even with the bottom of the hull.