go with the 496, no replacement for displacement!
If that doesn't work try nitrous.
Being serious though I am curious to find out what you decide. I have wondered a bit about this as well. Mike and Jparsons bring up good points. I know there are power mods available out there, but I have never known anyone personally who did it on a cabin cruiser. Sounds fun, but then it makes me think there is a reason why it hasn't been done