A little more food for thought - you could look at a motor like the Blata or Polini. They make between 10 and 12hp and have plenty of torque. They have been used in plenty of bugger builds and work fine with the gearbox and stern drives. You could also consider a clone motor which sell for a fraction of the price of the genuine. They are big motors but yours is a big boat
Here is a clip of an MHZ Showtime running the blata with what looks to be the same drive configuration you are considering -
http://www.youtube.com/watch?v=KR_pFo-HkcA.