Glen,
If the picture is anything to go by, it looks like you already have enough depth at the stern if you are ballasted down to waterline.
The only other thing I could suggest will cost money, though not a lot.
If it was me I would buy a couple of plastic two blade x series racing props of about the same size as the prop you have on now, one LH turn and an opposite RH turn.
These props are designed to have occasionally a bit of air around them and still function satisfactorily. They might also load your motor a bit more (shorter run times) but again they are designed for faster running, if you have in fact over motored the boat. Not pretty to look at, but they are not in view when the boat is in the water.
The reason for the second prop, is to try the boat with the prop in the opposite direction, and see if the problem occurs on the opposite side, if it does then this will show as Colin suggests, air is being drawn under the hull and it might be that the hull form is not designed for the power of motor you have installed. Also it was most probably designed for cruising rather than planing.
You don't say what year the plan comes from, but it just might have been copied from a much earlier design, in which case the required motor for the original design was most probably something that ran on 6volts, fairly slow revving, and had about the same power output as a standard 385 (if you were lucky).
I remember when the first 'high performance' motors were starting to be imported into the country, and they cost a fortune in those days, and all they were, were badly made and noisy 540's that sucked your, then wet celled, lead acid batteries dry in minutes. 'Orbit' and 'Jumbo' spring to mind.
Hope this helps (or maybe confuse you even further)
John