@RustyRatRod those are NGK ZFR5Ns, the best plugs for a stock/stock-ish pre-'75 Slant-6 head in many cases. The 'preaching' (ok lol it's more of a technical explanation but wtvr)
@volaredon mentions is
here.
@cruiser y'member those times it's been explained to you that the early Holley 1945s were crummy carburetors and there were a bunch of revisions and tweaks to them before they started being less crummy around '77? Well, you're looking at evidence. There was a TSB to install a "1 and 6 plate" in the early 1945s as a kind of crude hack to alleviate the № 1 and № 6 cylinders running rich.
'74-'76 intake manifolds were also, ah,
less than optimal for some dumb reason—the whole point of the buncha-bananas design was to promote even cylinder-to-cylinder mixture distribution, but then someone came along and "fixed" it for 1974, which aggravated the overrich front and rear cylinder condition. This was fixed for '77 and further improved later on.
