Over the years I have found the 68 to 70 inch 3 blade props to provide the best overall performance on the 912 series engines on airframes like the Kitfox. I know the longer blades like the 75 inch do better in the hard STOL environments, but they suffer immensely in cruise due to having too much drag. As an example, I have a 70 inch 2 blade Sensenich carbon fibre prop on my 912S powered Highlander, and it is a very impressive performer. It will climb solo at 1800 fpm and yet cruise at 108 mph at 5400 rpm. And that is with large diameter 8.50 tires and an open tube gear. For a Highlander that is not only a great cruise but also a great climb.

For comparison I tried a 72 inch 3 blade Warp with tapered blades with nickel leading edges and lost more than 20% in climb and 10% in cruise. I have a Sensenich 3 blade 69" (aka 70") I will be trying one day as I suspect that will be the best overall prop on my Highlander. Of course I'm not pushing my Highlander on mountain tops like you are. But even with my current 70 inch 2 blade prop I can outperform pretty much any Supercub. So I'm happy.

I think the 912 gear reduction providing such a low prop speed changes a lot of what we thought about propellor and engine efficiencies. That's the only logical explanation I can come up with.

Try to find someone in your area that will lend you a shorter blade to try to see if what I'm saying applies to your plane like it does on mine. That 75 inch WW is a VERY NICE prop, but I'd be surprised if a 70 or 72 inch prop wouldn't be more to your liking.