what fuel should I run in my 1968 GTO with 400 big block 4bbl?

I was to working in a gas station and it was .53 c a gal boy would it be great to pay that again

You can use avgas... 105 octane, or race fuel... 110 octane.

I worked at a gas station as a young man and regular leaded went for 26.9 cents a gallon. The best long term bet is to have your heads redone to accommodate leaded fuel. Additives may still be needed even then to boost the octane. Compression ratios were typically significantly higher back then. 10:1 compression ratio was not considered a high compression engine back then.

I know for a fact there was never an oil shortage at any time. I attended an algebra class in the evening at Penn Valley Community College in Kansas City Missouri in January 1975. The instructor for that class was an engineer for Phillips Petroleum. Several people in the class including my self found that out after class one night when we were discussing an algebra problem with the instructor. Someone asked him what he did full time during the day. He said "I am an engineer with Phillips Petroleum." Several of us kind of chuckled and made a comment about the oil shortage of a couple of years before. He looked up at us from his desk smiling, kind of snickered and said. "There was never an oil shortage, Phillips was hording gas to drive up prices. He also said that Congress took the price controls off crude oil production and set prices on old and new oil wells. Old well drilled before this date that oil companies had were only allowed to get $7.50 for each barrel of oil from those wells. Oil wells drilled after that cut off date and oil companies were allowed to get whatever the market would bare for the oil from new wells. Congress shafted all of us then and the oil companies have been sticking it to everyone since then. You can bet every member of congress back then bought as much stock in oil production that they could.

Beginning with the 1971 models GM designed their engines to use unleaded gasoline. Your GTO will definitely require premium with a lead substitute at each fill up. If the heads were rebuilt at some point and upgraded to be able to safely use unleaded gasoline the lead substitute would not be needed. I used premium with a lead substitute and octane booster at each fill up in my 1970 Pontiac Catalina with the V8 400. I had the car for 16 years and it always ran fine doing that. Personally I like STP products but everyone has their own preferences. Hope that helps! Jim