The early heads are not anything special, and from what I read, around '73 the seats started to get induction hardened. It wasn't till the aluminum heads came out that actual hardened seats were installed. The induction hardened seats are probably only the outer surface, not sure how deep it would have gone, so a well reworked head may not have any of it left. Date codes will be your best best to identify if they were factory hardened. If a machine shop cut out the seat and added hardened replacements, you need to actually look at them. They will be noticeable as a different color of metal ring around the vale face. I would bet any iron head after 1974 has induction hardened exhaust seats, all the lead in fuel was removed by then.
I wouldn't worry about them, worst case is it starts to miss, the engine is not going to blow up. Everyone I ran around with never worry about them, and we beat on the engines pretty hard for years. I have never had them installed any any engine I have built in the last 45 years ( mostly Pontiac ).
Joe, I can't thank enough for all of that information. It's taken a load off of my concern for the state of my valve seats, and for running the motor and driving without necessarily having to baby it all the time.
