Wayne,
With my OPS, I found that an Enya #4 worked pretty good. The OS R5 was too cold, as was a 'vintage' Rossi R5. I also tried a K&B 1L (what I used to run back in the day) but it was seriously down on rpms compared to the #4. Haven't tried an OS #8 yet, but it's a little 'short'. It'll probably run like the #4...

I'm running 5% Cool Power straight up with no issues. If you can set a good needle and have a consistent fuel system, castor really isn't needed in my opinion. I hate the gummy stuff to clean up anyway...

Regarding pipe length, here are the recommendations from my OPS pipe instructions:
15,000 - 16,000 rpm = 13 3/8"
13,000 - 14,000 rpm = 15 1/8"
Measured from the center of the cylinder along the header to the high point of the pipe (following whatever rise). Your results may vary depending on pipe volume...

FWIW, I'm at 14 1/2" and turning an 11 x 7 APC at 14,700-ish rpm on the ground when it hits the pipe. Its unloading in the air but not outrunning the pipe or falling off in long verticals.

Cliff

PS; Of course, this is all on a 35 year old engine...