I know this tread is old but I havent seen that you have done anything yet. Why not use something like this 3300SC Whipple Supercharger | eBay I know its not a bolt on for an 8.1 but a lower intake could be fabbed or Im sure they make something like it for the 8.1 marine engines. You would still have to make a custom rear inlet manifold for your throttle body and match the drive snout to whatever length you needed but with some enginerring Im sure it would fit.