If you are going to be installing a blower I would look for a 502 not 496. All of the research I have done concludes the 496's don't hold up well to running boost due to the fact that they have cast internals and not forged. Or you could pull the 496 sell it and get yourself a 540 which should get you to about 6-650 hp which in turn would put you around the 80 mph mark.
Remember it is going to take between 10-15 hp to gain 1 mph depending on the hull and other factors.

This looks liek it will fit your needs. It won't be as fast as you would like, but it has the look and seating yor after along with a mid-cabin..

http://cgi.ebay.com/ebaymotors/2005-...item4d0d8ffe1f

I know a guy with a cherry Advantage 250 citation MCOB with Volvo DPXr, kept inside with full cover