Why don't you post the last three of the casting number of the heads you have? Or if you have access to Mopar `Engine book they list all the numbers.
Big block heads got hardened exhaust seats starting in 1971. I understand why you are thinking this, but those are good heads. The problem is finding a buyer. I have several similar sets and can't find a buyer. I scrapped a VERY NICE pair of 1958 325 poly heads (Dodge) because I couldn't find a buyer...