408 spark plugs

I use NGK R5671A-7's on my stoker (with eddy heads). Reaching plugs 5 & 7....Ah yes, I vaguely remember those days. Sorry man, I switched to Headman Hustler headers years ago and have had no issues since...If I recall correctly getting a spark plug socket with a built in u-joint did the trick (got mine from Autozone I think).
Good luck.