What year 340 heads got hardened seats?

I think it was 72. They did not install hardened seats, the seats were cut into the iron head and then it was induction hardened. I have seen sunken seats on a 318 and a 340.
Lead helped prevent burned valves and seats. Another way to help prevent it is to cut the seat a little wider. The larger surface area helps transfer heat from the valve to the head.
Mopar did tend to use better iron than the other two brand X. Ever pulled down a 100k+ mile 302 ford? Every one I ever pulled apart had a massive ring groove at the top of the bore. I have never seen that on a 340, which they did advertise as having a higher nickle content. Pulled down a 40k mile 318 and it still had the factory crosshatch in the cylinders. So, metallurgy might play a role in why you dont see sunk seats on the mopars very often....but it does happen.