I put 100K miles on a set of '69 C90E heads with no seat problems, all since the demise of leaded fuels. The only head I ever had trouble with was one camel hump Chevy, and that was after years of use. It pounded out an intake, I just reground the 1.94 seat to take a 2.02 valve and drove it 50K more. The whole hardened seat thing is highly over-rated, at least in my experience.