Ryan,

The Reivers prop calculator; when 1 HP is assumed at 10,190 RPM, a 12x7 prop can be spun at this speed.
I may even be optimistic.

If a large prop offers enough load to put the engine below its peak torque, it may even spin more slowly.

I guess someone who has this engine, should tach a 12x7 and tell us all the results and on what fuel they were obtained.