Spark plug heat range.

Trust what the engine is telling you.
Aluminum heads suck heat out of the chamber pretty good.
I have those same plugs in my Eddy-headed 360. They've been in there for well over 100,000miles. I bought a new set years ago, but when I pulled out the current ones, they looked so good, I screwed em back in; again and again..........
Not the same,I know, but ...........I trust the plugs.

BTW, I ran my street 360 at 205psi on 87E10 just fine at 34*power timing. Like said, aluminum pulls heat pretty good(bad). Ok, hard.