Just a Sneak Peak......

[spareparts]

get out the credit card
http://www.stainlessmarine.com/index...emart&Itemid=1

you can probably start off with just installing thru hull exhaust tips http://www.rexmar.com/page230.html and connecting them to the existing risers with 4" exhaust hose. You will need to block of the stock exhaust Y pipe. To do it right, you need to pull the engine, remove the Y-pipe and install a Mercruiser block off plate

If you haven't already found them, look at Eddie Marine http://www.eddiemarine.com/ and Rex Marine http://www.rexmar.com/productindex.htm

[THEFERMANATOR]

Heres a few pieces I found on Ebay you could use for adding in through hull exhaust without pulling the engine.

Heres a used set of CORSA electric diverters so you can select between the regular I/O exhaust or uncorking it when you want to.
http://cgi.ebay.com/ebaymotors/Corsa...ssoriesQ5fGear
A set of 4" through hull tips.
http://cgi.ebay.com/ebaymotors/4-STA...ssoriesQ5fGear
A set of basic diverters that lets you use both.
http://cgi.ebay.com/ebaymotors/BAJA-...ssoriesQ5fGear
And a diverter and through hull set with mufflers and flaps.
http://cgi.ebay.com/ebaymotors/Corsa...ssoriesQ5fGear