A full heat range may be too much but it would be worth testing if you want to narrow it down to a nats ***. Start with idle tuning and see if it cleans up the soot.
I will try to cut a couple of the spark plugs to see if i can get you guys more info, 5 of the plugs are a touch black mid way down the porcelain but cylinder 7 and 8 look the same and the porcelain is pretty uniform as deep as i can see