I have many many miles on original engines in old cars and trucks. What i have found is they are fine until the valve seats are ground, which removes any residual hardening or lesd. Next the fresh valve job does not last long because the valves sink into the soft cast iron of the head. Having been through this several times, and having to pull the head/heads for a re-do, I have become a believer in hardened seats. Original valves themselves can usually be reground as long as they are straight. I can't imagine doing this without doing a complete job while I'm there. Valve guides are knurled, springs replaced, and I like to have PC seals installed. I've had much better success with them than the old umbrella style seals.
Devin
