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Your engine has a 2.33 gear ratio, and according to Mercury propeller charts and information, you would need a 17" pitch for heavy load carrying, or a 19" pitch for lighter loads. Your top speed will be around 36-40 MPH. The engine redlines at 5250 RPM.

If you are in the aluminum price range, the best you can buy is a Mercury 4 blade Spitfire (also sold as a Quicksilver Nemesis). These are the best performing aluminum props made, and give good acceleration and low speed planing. I would try 19" pitch.

If you want top of the line SS, Mercury has recently brought out a variation of this prop, called a Spitfire X7. Cost is about $390 delivered. Mercury is claiming this is the fastest and best performing propeller available for any 70-125HP engine, period. I have not yet tried one.

The mid range Mercs have a "taller" mid-section height than most engines, so I would mount the engine in the third (middle of five) hole.
That is all the height you can use or need for a low HP engine like a 70.

You can research the Spitfire props at mercurymarine.com, and find good pricing for them on-line. Also search for the Nemesis, which is identical but sold on the after market.