340 Chugging instead of Reving

NGK. The engine I had this issue with was an iron headed, high compression, stock stroke 360. Ran the old N9YC plugs for years with no issues. Put in the NGK at the beginning of a season and had the issue. Had replaced the tune up parts as well, in preparation for said season. I replaced EVERYTHING trying to find my miss. Coil, distributor, ECU, plug wires, etc. Plugs were last on the list and guess what? Lol.