It's just the cheap iron used by Chevy back in those days.Sounds like a bad valve adjustment and the wrong oil.
Definitely use good lifters like Comp Cams. No sense in putting in a cam and ditching 40+ HP for $50 more on lifters.
Was VERY common for flat cams at about 50k miles.
When I acquired it and pulled the valve cover to see what was going on, the valve cover had never been off before, everything was still shiny new.
Not an oil issue
Are you seriously asking that ?280-292 at .050?
Something like this...

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Figgers, right ?
Someday I'll get to it.
Could honestly give a fuck.....but if someone offered me the right amount for it, it'd be gone, like, now.