Yay, Saint Sheldon to the rescue.
https://www.sheldonbrown.com/tooltips/bbcups.html

Fixed cup was being a pain.
So 4 days of penetrating oil, then a 5/8 bolt.
Applied the socket set and my 4 foot scaffolding pole cheater bar.
It just sighed and gave in

No damage to anything.
But with my tiny desk vice it was tricky getting the bolt off again.

Weight now: Frame 1930g, fork 745g, total 2675g (5.9 lbs)
That's my biggest frame, oldest frame, and lightest frame.