Mechanics and Fixing Any Questions Answered

https://uniortools.com/eng/product/1607-4-bottom-bracket-shell-installation-tool#19816

This is the tool you need. Used to get ones from cyclus tools or someone like that made them decades ago. Unior now make this beast which so far has rescued a whole bunch of frames from my scrap pile. The two very chunky pieces of steel thread together sandwiching the cup against the tool face (has two slightly different sizes), as you undo it, the tool self tightens against the cup ensuring no slip. Then simply add heat, kroil, tappy tap hammer and repeat until it shakes loose.
Used to have a very oldshool 'D' shaped spanner that fitted over these type of BB, but someone trashed it by just sending it instead of add heat, kroil, tappy tap hammer and repeat until it shakes loose.