A 4 blade will not increase speed, it will take away from top end. Since it is a 76' model and does have the 2.41:1 gear case and RPM range 5500-7000k I would recommend a 3 blade 10 pitch prop if your putting it on a 12' tin. This would probably be a safe starting point to get some info. From there you can adjust in pitch to get optimum performance. But you have to start somewhere and cannot expect to get the perfect prop on the first try with out any data to go by. All we can do is give you an educated guess from our experiences.
If you can turn a 10pitch at 6250-6500rpm you should see numbers around 20-22mph assuming a 15%-18% slip