Plug chop!

It seems like you had put in an 8.5 PV last time something was posted.

Your stock secondary metering plate has .067 holes for what would be the "main jets" for the secondaries.

The plate number is 134-8, you should be going bigger there too. You can get one like a 134-39 that has .073 holes that would definitely add more fuel to the secondaries

With the 4160 you either need a new plate like above or a conversion kit to a secondary metering block or a metering plate that you can screw main jets into instead of changing plates.

Those look like NGK plugs, what's the number on them?

Your engine isn't exactly radical so it doesn't seem like you need to be dumping in massive extra amounts of fuel

Here's an adjustable secondary plate that I think would work. Key word is Think

adjustable plate

How does it run?

Is it dull at a particular throttle setting?

Is there a lean surge in any portion of the operating range?

For a plug chop to really mean anything, you need to first disable the secondaries and run wot for a spell, then cut the plug.

After sorting out the primaries, only then do you move on to the secondaries

either those plugs don't have enough run time on them, or , well, if you were that lean to not have a soot ring, there would be drivability symptoms of some type, which we appear to missing that data.