One thing that I see missing in this discussion (and I don't claim to be an expert on this) is that how octane ratings were determined changed around 73-75. Before that producers has their own systems for making octane claims.
An interesting discussion of how octane rating changed from RON to MON. So comparing old and new octane numbers is like comparing gross and net engine horsepower.
And in the US its been standardized as the average, also refered to as Anti Knock Index (AKI).
Per the query on the Oldsmobile forum, my parents had mom's '69 318 Belvidere tuned for American Premium (Super, unleaded). In other words the timing was advanced a little. Was this common? I doubt it.
All race fuels are expensive. Any fuel sold for street use also has road taxes added to them.
I bought 5 gallons of GTX at 10.50 per gal this spring and considered that a good price.