To get a better top end, the easiest, most reliable way, and often really the only way, is a bigger engine. A stainless prop is not likely to do much, if anything.
To get the most from your current set up, a prop that allows your engine to hit the rpm where it makes its peak hp will yield the best speed. Propping above or below that rpm, even though it may be within the WOT range will result in a lower top speed.
You are using a gearcase with a lower numerical ratio(taller) so to compensate, a prop with lower pitch will be needed to get the rpms back to where they should be. But with a lot of props as pitch drops so does efficiency.
Since you haven't provided any info like your engine WOT rpm range, what rpms you were getting with old gearcase, and what pitch was the prop on it, it really is difficult to make any recommendation.
The ratio change results in a approximate increase of propshaft rpm by 18% relative to engine rpm, so a decrease in pitch by about 17% from what was on the old drive would be a reasonable starting point.
Getting rid of unnecessary ballast will help. Wife and kids, the Mother-in-law, especially the Mother-in-law are to left ashore or tossed overboard. Mistresses and Girlfriends are allowed to remain board.