I believe in 1974 or 75 is when all the emissions crap came into play. Pre-74 engines will be a lot faster stock because they didn't need to run catalytic converters which made it too expensive to do dual exhaust, and they didn't need to tune their engines to run emissions friendly instead of performance minded. I'm not sure about this, but they could probably still run leaded gas considering there was no O2 sensor or catalytic converter.