I read an article about these engines in Poker Runs America magazine sometime back. The HP3 is a high performance version of the 496 that are on the order of 500 or 525 horses as I recall.(?) I don't remember the specifics... I think the HP1 and HP2 are the 385 and 425 HP versions used by Mercury.
They were designed by GM as marine engines and I don't remember if they had any design assistance from Mercury, but I think not. I do remember comments that this being a product of the GM production engine group, they were competing directly with Mercury's high performance group, builders of the HP500, 525, etc.