I have an 1978 Fairmont GXL XC 4.1 with Extractors, bottom stock end, an Alloy head (XD or XE not sure) ported and polished and port matched to triple 45 DCOE Sidedraft Webers, I was running NGK BP6E S Plugs (I think these were for the cast iron head and probably didn't seat properly and ran black, along with backfiring), I have just put a set of NGK BP5FS plugs in (supposedly correct), improved already, but seems confusion on what is recommended, any assistance appreciated.