Hardened Valve Seats or Not?

what year are the heads? check casting date on top, 1972+3 had hard exh seats, 1971 maybe. engine is assembled, and in a not heavy car - run it. heavy loads and boats need hard exh seats. my 1970 Duster 340, 1.625" manley valves, flat solid cam .555" lift and .022" consistent lash street and strip all good since 1995