At this point it could be a lot of things including a bad ring or cylder wall and for that matter a broken intake manifold with a leaking runner.
We could start a pool and bet on what we think it is and give a prize to the winner.
It's just at a point where the only way to find it is to disassemble.
There are lots of strange things happening. For me, intake gaskets are not matching up well. Early intake manifolds are not fitting up well to later blocks and heads.
I've got a thermostat housing leaking fluid and the only thing left is that the housing itself is porous and leaking right through the casting.
Some items like that never had any consideration of still being in use 50 years later.
Although a ring issue is a possibility, the quantity of oil being consumed doesn't line up with that. We're dealing with probability now, not possibility because literally anything is possible.
That no longer matters. The top or the engine has to come apart with AT LEAST the intake coming off.
I'd bet on the intake manifold misalignment to the ports on the heads, but that's just the latest guess. I just did my own T/A intake install and it's very fair to say that the intake gaskets were an issue.
Third time install was a charm...except for that thermostat housing. That's not done yet.
Two sets of head gaskets. Issues that didn't exist 30 years ago.