I made the calculation and you have a slip of 16.5%. I write in my post the numbers I run last time with best of 74.4 mph @ 5400 rpm with a Tempest Plus 27 and have a slip of 6.2% .
If this is your max rpm, with a 26 pitch and bigger diameter prop like the Tempest Plus with a slip of 9% . You can run 85 @ 7100. This is only a theorical calculation based in the numbers I have. Your gearcase run higher than mine, but the perfect slip should be less than 10%.