How much does it cost to rebuild a Slant 6

451Cuda said

Find a later model head if possible, will already have hardened seats installed. I think it started in '73?

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I can't say for sure about Mopar, but the others of the Big 3 used Induction Hardening of the iron head to create 'hard' valve seats. They did not use any sort of insert seat. When you see that done it's because a machine shop did it and someone paid for it. Locally that runs about $300 for one V8 cylinder head.

The problem with induction hardened valve seats is that hardening isn't very deep and usually the first valve grind will grind thru it. Then you're back to just an iron valve seat, all of the hardening is has been ground away.