I ran the OS 46 VF in a 40 size MK Arrow pattern plane, with a Macs pipe and header. As I recall, the header length was around 3", with a quarter inch gap between the header and pipe. This set-up turned 15,500 on the ground with a Zinger 10-7 cut down to about 9 3/4" as I recall. In the air this would unload to its max hp rating of 17000. Absolutely awesome performance. I flew it in Advanced pattern one fall just for kicks, and it was running circles around the piped 60s in the go fast pattern days Oh, this was on 10% Cool Power or Red Max.

I think your prop sizes mentioned are way too little for this engine.

Clair
AMA 15654