When hardened valve seats production??

OPs original question: When did the hardened valve seats go into production in the slant 6's?

Answer 1972 model year.

Without a catalyst, cars with the hardened valve seats and otherwise set up for unleaded fuel, ran leaded gas with no problem. Given an steady diet of leaded gas would lead to more deposits on the plugs than a steady diet of unleaded. FWIW: Amoco made a super premium gas that was unleaded. Supposedly a "hot" batch of this stuff would have 102 octane. It was Amoco's top-of-the-line fuel and available at nearly all stations.

The unleaded fuel that was available was 87 octane (IIRC) and did not run well with performance tuned high compression engines. Furthermore when catalytic converters started appearing in 1975, engines started sounding like Hoovers on speed due to the poor catalytic converter pass-through of exhaust gas.