I'd agree with this but I have doubts that's the problem as I replaced the original heads less than 10k miles ago with rebuilt late-70s castings I bought from a fellow FABO member that had stainless exhaust valves put in. I guess I could still check the stem heights at some point. I'll be pretty ticked if that's really the issue.
You never mentioned you replaced the heads. I believe you said it ran good for 133k. If the heads you installed were surfaced your pushrods may be to long. Or who knows what condition the seats are in the heads.