What do you think of this plug?

yellow rose said
I try to keep them at .030-.040.

That's good then.

yellow rose said
The power valves are 8.5’s but I’m changing them to 10.5’s.

If I go smaller on the main jets, it gets a lean surge at cruise.

Jeez, i ran into this with a chev 383 I changed the secondary jets two sizes smaller. When both both 4 barrels were opened there was just too damn much fuel, the only other issue the guy had was float level too high & I mean just a mm or so. Do your carbs have removable air bleeds ?
Is this a race only engine? Or do you cruise it? 1500 cfm sounds like over carbed for street duty. What does your exhaust smell like? I'll go out on a limb and say just from the looks it's being washed a bit & the black under where the threads are is unburned fuel & carbon from exhaust reversion . It's just a guess, if I could smell your car & see it I could nail it, I'll be over in a few weeks hopefully for good. Great at idle, good at mid rpm and most likely over rich at wot that's where I think it maybe cleaning your porcelain