What year 340 heads got hardened seats?

We had unleaded gas in the early 60's AMACO High Test was unleaded and there were no head problems .I have been running unleaded in my cars for hundreds of thousands of miles with no ill effects .I think this whole valve seat thing is a load of crap unless you are really beating your motor to death constantly. Besides,sll MOPARS have flame hardened seats from the factory so unless you had them ground you are fine.