Carburetors don't change for leaded or unleaded gasoline. The mechanic's claim of avoiding valve seat recession by running a richer mixture is pure unadulterated hogwash. Adding Marvel Mystery Oil or whatever does NOTHING for the exhaust valve seats. It does make you feel better because you have "done something" but you might as well have paid a medicine man for a special charm to hang on the rearview mirror.

Chrysler didn't start induction hardening seats until I believe 1973. I think GM did it one year earlier.

If you are lucky enough to have a real hot tank around, when you get your cast iron heads back there should be a blued area around the exhaust seat, maybe both. This is the indicator.

If you are using a Neway or similar seat cutting tool, the hardened seats have smaller chips. It's quite easy to tell. If you're using a stone I don't know you'd see a difference.

If you are spending big money on your heads for sentimental reasons, then install the hardened seats. I learned something new today...I had thought the seats were always installed by cooling them before installation. I would agree that using mechanical force could be more problematic.

My '64Dog's Poly 318 ran well over 130K miles on unleaded fuel with no valve seat recession, but it had the stock springs and camshaft. It has on occasion run for miles wide open pulling up a mountain,, which is the hardest on the exhaust valve seats. But those mild lobe ramps and soft springs were easy on the parts.

With any cylinder head work, get a firm written quote before the work starts, and compare with a new set of aluminum heads. Many times the cost difference is small. In some cases the aluminum heads may cost less than reworking your old heads.

R.