RE: os 61sf
Dave,
The OS.61SF, like the other long-strokes of its generation, is even more bottom-biassed that that.
If you prop that engine to spin between 10,500 and 11,000 RPM ground static; and adjust the pipe to an appropriately longer dimension, the engine would produce that target HP.
11,741 RPM is still a bit over the hill, although it is less so than Tony saw with the 12x6 prop.