One of the real mysteries about Mercruiser Big Block Engines is that you can get a 485 HP 454 but they start at 300 HP ! it just depends on the combination. The 496 is rated at 375 or 425, for my money I would have the 454/485 bullet cos I can claim the smaller cube disadvantage. Now it all depends on what you have "now", head port size, Injected or carb, how good is the fuel supplied at the pontoon. It's not just slapping all the bling on, it's where you require it to work.
Really good engines take hours to build & even more hours to spec the combination that will work. If you want 850 HP, can do that but they need rebuilding every 4 hours & 113 avgas, not really practical !
I would stay with your original concept as you will really notice the upgrade in capacity. Some of the surrounding parts may have to go, your distributor is definite. The carb & inlet may be adequate, along with the risers, as may well the valve train do the job. I need to know what the original engine was spec'd at. 300 to 485 ?
No I do not have a 496 for sale.