Goes to show how much I know about OS plugs... I'd never heard of the #5. According to the OS site, they now produce a #10 (cold) which was formerly the A5 (this one I was aware of). There is a racing 5 too (R5) but I suspect that is not right for 60 size Speed/13 (better suited to smaller engines turning much higher RPM).

That makes sense though, you would want a colder plug on this beast unless you shim it up slightly on the hotter plugs. The #8 is considered a medium plug. The Speed/13 is a long stroke though and I believe the #8 is what OS recommends for their SF/RF long strokes. Then again, the OS's likely have lower compression (and certainly lower rpm) per design compared to the Speed/13.

Wherever you bought your NR, you should also be able to pick up some cold C6S plugs as well. Other interesting options (which I hope to try) would be the AXE Rossi plugs, R4 and R5. I don't expect substantial difference between the right AXE plug and the right NR plug given the environmental conditions. The PM 15% should be OK but you might want to add some castor to it if it is all synthetic which I think the 18% is. The 10% GMA (CL) with 22% oil might be a better fuel while the engine is new. NR recommends 10/10/10/70 fuel for the engine (hence a 50/50 Castor/Synthetic mix).

I look forward to the pics and video of your run-up. I don't think we have any public video of this engine yet (at least not that I'm aware of).

The Super Arrow is a beautiful classic.

David