I will probably be in the minority here but the cost of new exhaust, mounts, carb etc. will go a long way towards building a new mouse.  When I was a teen our mostly stock 400 SBC had enough grunt to pull up 5 skiers at once.  I don't think it ever turned 5000 rpms and it sipped gas.  Some pretty crazy HP levels are pretty ez to obtain with mouse motors nowadays.
But I'm sure the rat moto lovers will feel differently.

I would agree. I would probably build the engine as a 383" stroker and freshen up the pump.

I would take an educated guess you are likely to find a B impeller in that pump, regardless of what the tag says, if it was set up as a SBC to begin with. That and/or the weight of the boat may account for the RPM differential you are talking about. 

Now, if you are convinced of going to a BB, going to a 454 would be far easier than a 460, however. Your driveline and bellhousing/rear plate (depending on 3 pt or 4 pt mounts) can be reused, as well as possibly the flywheel. you would have to replace and relocate the front plate or side mounts (again, depending on 3 pt or 4 pt mounts) and then play around with crankshaft centerlines and driveline angles.

Nordie changed his old boat over from 460 to 454 awhile back. Don't think he wants any part of doing THAT again. 

Ray

Maybe I'll start with the pump and keep the SB. Then like you said I will most likely find a B inside. The boat did come with a SB from Daytona in 85.

I thought about the 400 deal but the blocks are hard to come by from what I hear. Though I haven't looked around much either so I could be talking out of my ass. It sounds like everyone knows and believes building the SB is the better choice.