props and gear box ratio

There is no advantage in turning a prop Faster.
The Slower the prop turns and the Larger its diameter; the better its efficiency.  Think, V-22 Osprey!
It looks like you have plenty of clearance for a larger prop.

i have a 72 in. prop now and that's as big as most manufactures make .going from a 3.0 box to a 3.4 would be better . thanks again

everything else remaining the same what would a 75 ' prop do for performance

You can get too large and then it becomes a weight and balance issue wearing on the gearbox, ground clearance and prop pitch efficiency. 

There are upper and lower limits for many things.