When our '66 Newport Town Sedan (383 2bbl) was new, it clattered on normal regular gas back then, although it was spec'd to run it. So we used premium as a matter of course. This was in '67 and later (we got it when it was a year old).
When I started driving it to college, I continued to use premium as every time I tried anything of lesser octane, it clattered on acceleration. I was also able to advance the base timing from 12.5 degrees BTDC to 15 degrees BTDC with no problems.
Regular grade gas, back then was 95 Research Octane fuel. Premium was 99-100 Research Octane fuel. A pump octane label of "95" would be the same as 99 Research Octane, by my calculations back then.
In reality, set the timing to specs (12.5 degrees BTDC) and see how it runs on "93" pump octane fuel. Then back it down a notch to "midgrade" and see how it works.
Although all of the engines were built to the same specs, there was "a range" of allowable power output from each of them. Some worked with "regular", others needed "premium", by observation. Don't worry about what the factory specs were back then, just use what works best in YOUR car for the way you drive it.
"Research Octane" is NOT what the "Pump Octane" number is. Back then, many knew the difference (what the (R+M)/2 equation represented), but probably not that way anymore as that's the only number many NOW understand or know about. I believe there's a decent explanation in Wikipedia. ALL of the service manual/owners manual literature from that 1965 era uses "Research Octane".
Also, in those earlier days, every major gasoline company distributed their own gasoline, typically. NOT that way any more! A contract carrier pulls up and dumps their gasoline load. They picked it up at "a terminal" and brought it to the designated station. So, as long as the brand is "Top Tier Fuel", that's all you really need to know, regardless of which brand it's sold as, best I can tell. Things are much more generic now than in prior decades, but DO buy the "good brands" rather than "no-name" stuff.
In those prior times, some brands just seemed to work better in some engines than in others, too. Things you could feel in throttle response and whether or not it trace-rattled on acceleration. THAT much difference, even with the name brand/major brand fuels, by observation. And, of course, all of the electronics on modern engines have numbed how things work with various fuels, too.
So, find what works best in YOUR car, starting with "Premium" and working down in octane at the factory settings. If you might be able to get it to work well by backing the base timing back to 10 degrees BTDC, with the "Mid-Grade 89" pump octane, that might work too if $$$ is important.
Key thing is to have it tuned as well as it can be tuned, for best mpg, at which ever octane level you end up.
CBODY67