AC Delco Rapidfire #4 plugs in a slant six

In 1995 when they were newly released, I tried those RapidFire plugs in my '65. Got a bunch of ping and removed them to find tiny globs of metal on the tips (yep, ping). Never tried them again. Use NGK ZFR5N (stock #3459). Remove the metal ring washers whenever installing plugs (of any brand) on a '63-'74 slant-6 head. Tune-up parts and technique suggestions in this thread.