The cheapest option to try would be to see if you can take some cup out of the prop and/ or de-pitching it. I suspect you can't pull anywhere near redline without significant slip, so you might de-pitch and keep top speed?
No slip speed for that pitch looks like 54MPH at 5000 RPM with that gear ratio. So it looks like you have room there, unless you are getting north of 45 MPH now?
That and polishing and making the leading and trailing edges more fair are where I would start. At redline, the prop tip speed is pretty high, maybe 220-230 ft/second, so you are already starting to walk a line between blade loading and tip speed.
I think a different transmission ratio with more reduction would be a better option, but prop tuning is way cheaper.
See what a good prop shop can do for reducing pitch and getting a fine edge on the prop. An inboard prop is better protected, so a slightly fairer leading edge is probably ok.
If this works, great. If not, it tells you which direction to go.
If you have the money, ACME in Big Rapids, MI makes nice inboard props.
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