In '67 you could buy Sunoco 260 for about $0.23/gallon in my area. By todays calculations the octane rating would be about 98 {(R+M)/2}. It contained lots of TEL which was deadly and would foul plugs in about 5 minutes if your carb wasn't set up properly and if it was, they would foul in 15 minutes at idle. But it protected your valves.
That's why your '67 would run on premium back then and not now.
Stewart
PS: I used to run it about every third or fourth tank to run it hard and "to clean things out".
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1976 TA, nose converted to 1970 style, 406, ported #13 Heads, '70 iron intake without crossovers, Q-Jet - Cliff style, RARE OS manifolds, Pypes duals w/crossflow, UD 230/238 custom HR 4/7 swap cam with solid roller lifters  , Hydro-Boost 4-wheel discs, 4 Speed, 3.23 posi.
“Nothing in life is so exhilarating as to be shot at without result.” - Winston Churchill
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