What octane should I need?

I'd say put 1/2 a tank of pump gas in it, swap the plugs, and see what the new plugs tell you. I know the difference between the "beef" in a MacDonald's burger vs. a ribeye, and to me that's the difference between pump fuel and race gas. Av gas would not be an option unless the car was driving out the back of a plane. It sounds like you know what to look for and are on top of things. You can probably get away with it, with the understanding that the engine will tell you if it isn't working. You may need to tweak the timing curve, and potentially the carb depending on how much ethanol is present in your local mix.