Olnolan already suggested a blower but you have way to much compression for that. Dreaming a little, it would however give you an instant almost 100 hp at the start and probably 150 to 175 top end with no rpm change. You probably aren't going to go 10 mph faster but you will be in the rarified field of fastest Natuque's around. The starting blast would be great. I doubt a person could hang on to the handles on a barefoot start!!!
You could probably put on the larger high pitch prop and go a little faster with no change of your top rpm but you already know that boat drag is exponential.
Reality says that the RPM manifold and about an 800 vacuum sec carb with a yellow spring would be the way to go. In this case the Air Gap would work too as the outside air is usually balmy and your under hood air temp can get pretty high. You won't really need the heat passage.
Keep the rect heads. They are helping you.
I'd just freshen the motor and do your cam what ever you decide. I think you have a pretty good idea of what the boat needs.
Once again I'd suggest an AFR gage to get the tuning exact. You will need the data logging feature too as your runs will be longer. You will need a good Holley book to understand exactly how the carb sections work with each other. You will be amazed how much you get by just getting the carb really tweaked to your operation. You can make adjustable air bleed with brass set screws. See the book.
I'm not a big nitrous fan but for the relatively short bursts and for just wasting fuel and blowing rooster tails and general screwing off it would be hard to beat. Even a 50 or 100 hp shot would be just plain fun. With a boat weight isn't a real big issue so you might even consider getting the whole bottle instead of just a couple 20# ones. Hard to beat $$$ per hp.
Don Hampton blowers is really big in boats if you still want to go after a blower. I think you can just flip the front cover around on a 6-71 and drive from the right side instead of the left for rev rotation. The rotors would still turn the same direction. Install the idler system on the left side of the motor. I haven't done this however.