340 X Head Value

First time I heard of this was a friend's 318 challenger developed a bad missfire. Turned out to be bad exhaust valve or valves. Then a few years later a 340 with X heads I owned had been rebuilt without any attention to hardened seats. Not daily driven but seasonal driven developed a missfire and turned out to be sunken / bad exhaust valve seal. Repaired with hardened valve seats and have been happy motoring ever since. Approximately 10 years and 30 k miles ago. Just my experience yours may differ ??