I'm using 1 in my Gee Bee R3. Swing a 20x12, RPM a little over 6k. Lovely engine, very smooth. They come with an Rcexl CDI and CM6 plugs.
There earlier release had a short prop shaft. This was a pain when using a spinner with a backing plate. Seeing they just use a single nut on the prop shaft, you ran short of threads for shafty. The newer ones have a prop shaft 10mm longer.
I hope this helps.

Cheers
Grossy