Outboards are my weaker subject but I would recommend a "prop calculator"
They are all over the internet and you type in your boat weight, size etc and engines and it will recommend a prop for you.
Don't know how deep your drives dig into the water, but you might want to try a pair of "Clevers"?
I tried clevers on my 260 but my drives were too deep. The boat was lazy gettingh out of the water but the top end was incredible!
You may also consider the differences between a three and four blade prop too. Talk to a Mercury dealer. I did and he helped alot when I was playing with pitch and diameter.