LA360 initial timing too high - are there different timing covers?

Did your gauge have a solid seal around the spark plug hole? Dang.... that IS looooow. I think you get some sort of prize for that.... Is this with a good compression gauge? (You mentioned not trusting yours.) DCR is coming in at the 6.3 to 6.4 range with GOOD cranking compression, based upon a 280 cam at 110 LSA and 106 ICL. That would be around 120 psi cranking.

Knoxville itself is not at all that high an altitude. Are you way up in the Smokies somewhere at 3000-4000 ft? I doubt that is the issue....

You have effectively a whole point less of compression, so either things are pretty worn out or that cam is pushing 310 degrees duration! Which sure as heck is not a MILD cam LOL. OR the cam is reeeeely badly retarded. It would take an intake closing angle of around 80 degrees ABDC to drop the stock SCR to that level of DCR (around 5.6) and get 100 psi cranking pressures with good rings.

I think you need to re-do your cam timing first on the lifter body, and see what you really have cam wise. Or see if you can find some ID on it. And it won't hurt to do the other side and record them all in detail and try another compression gauge.

BTW, I recall a Vega engine 'back in the day' with the usual really worn cylinders, and it would take 30-40 degrees of ignition timing advance and would idle noticeably better.