I know quite a lot about the HP3 engine. Dave Stiff at Innovation Marine (941-355-7852) does as well though most of the people there that worked on the HP3 project are gone. Last I heard Innovation still had a stock pile of the original 87 octane HP3 engines on the shelf though that GM supported race program is long defunct.
I basically attempted to convert a 2004 Mercruiser 496HO (425hp @ 5200 rpm on 87 octane) to a HP3.
What specifically do you want to know about the HP3?