RE: O.S. 46 FX on a pipe
I notice the pressure tap in the header. The factory tuned pipes I've seen place the tap at the fattest part of the pipe.
After taking a good look at your photos it looks like the header is already shorter than a new one from Macs..but that's my guess from memory. You might find that tach readings go down slightly if you slide the pipe closer to the header with the 10x6 prop..?
Regardless, I don't think the 10x6 has any future on that plane if you are trying to make it run as fast as possible, so your relevant tach readings during your tuning session are made with the 9x7 or 9x8 prop.