I don't think you need to change the ratio. I would be pretty certain of the pitch prop you need before paying the big bucks though.
Here is a little tool that should help...
Boat Prop Analyzer (http://www.boatramp.com/prop_applet/...erAppletG.html)
I have the same boat with a 94 350 magnum and 1.5 gears. I use a 21 pitch high 5 prop.
Using the analyzer looks like you'd need more like a 25" pitch to make all things equal.