Yeap this is the boat i have.My run close to 70 mph.

fixx
 

if you can figure out what your upper gear ratio is you may be able to get away with just a imco lower..

ratio is 2.0 as far as i remember , what does that mine...?
Which one would work for me ,there's many imco ...
Sound's like a plan :)

fixx
 

2.0 means that the input shaft of the drive turns 2 revolutions to the props 1 revolution..

first you will need to draw a line center of the inside prop shaft on your drive thats on the boat now..lower the drive so its even with the back of the boat and see how many inches it is below the bottom of the boat...

-2 is a common lower to go with..keep in mind you will need a 4 blade prop and the boat will handle different then your use to..it will be allot looser..

http://www.imcomarine.com/cal_store/

At that speed a Bravo I would likely net you the same thing as 30-50 extra horsepower...2-3mph tops, but then you would lose all the advantages you get with a Bravo III, I say leave it alone. For reference a similarly sized boat, by brother had a 454 232 Formula Bravo III and put a Bravo I on to try it out, he wend from about 61.xx to just under 63mph (less than 2mph) once he found the right prop. He changed it back since the B3 was so nice around the docks etc. Obviously the faster you go, the more it slows you down.

If there is an easy way to go from your motor to the HO and do the drive it may be worth it (similar to the cam and ECU tune on a 496 standard to HO), but I sure wouldn't go hacking into something that pretty. And I don't know anything about these things, but I would think somebody would know the difference here.

EDIT: I see you are talking shorty lower, that will obviously be worth more than just a standard bravo.