Go to www.marinepowerusa.com. (http://www.marinepowerusa.com.)
Pretty good site, their forum sees little action.
The 454Mag/Premium motor is definitely the one to get. It can make a good base for HP downstream. The std engine is cast everything and does not want to survive with substantial HP boosts.
If you can, check out their X-motors.
Stay away from the new 8.1 block motors. The ECM is not calibrated (yet) correctly for the fuel mapping demands of jet drives.
From what I understand, the 8.1 is fine in Mercruiser I/O's.