587 heads

what year are these heads? what year did mopar start hardening exhaust seats, around 1973? my heads were machined for larger valves quickly on a Serdi machine at 30 45 60 70. we found that a heavy truck or boat needs hard exhaust seats . a car with quality stainless valves not so much. my 1970 340 with Manley racemaster valves and solid lifters is still good around .020" lash since 1995