Ray, I can't tell your exactlly what year they went to hardened seats but if memory serves me right the big change to unleaded fuel was 1973. Remember the year when all cars had so much polution garbage and the timing was retarded so far that they had no power. I beleive it was in that time period (73 to 75) that they went to hardened seats.
I don't think you should waste your time worrying about it though. Have you ever had anyone tell you they wore out valve seats or guides prematurely because of unleaded fuel. That was a problem that never happened. The automotive machine shop that does my work for me told me that every garage was expecting lots of head work in the late 70's and it never came. I had a head for a 200 cu in. 65 Mustang done in the late 80's. I asked the shop if he put in the hardened seats. He told me Ford never changed the part number for the seats from 65 til then. Put whatever head on that fits and have fun.
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Gael
37 chevy sedan, 261,t5
57 pickup
58 burb
