Ouch, that's nasty. I had a similar problem trying to get the BB fixed cup out of my 1972 Atala when I converted it to fixed gear. It's Italian thread (of course), so whoever installed it made sure they got it in plenty tight. Plus, instead of sensible flats like Campy used on their fixed cups, this one had notches like the adjustable cup lockring. But not in pairs so a properly designed lockring tool (like the VAR #16:
could grab it. No, this one had 3 notches, so only those cheesy single-tooth lockring spanners would engage. Needless to say, I couldn't loosen it with such a tool. Of course, the cup was nicely hardened steel, so sawing or grinding a new notch would be a royal PITA. So I made my own notched tool to pull it out:
Failed. So I took it in to the LBS for them to try. They had it for a week or so, tried everything they could think of -- penetrting oil, heat, cold, BFWs, etc. No joy. I took it homeand it sat for a while, when I finally found this little number made by Hugh Enox:
A couple BFWs and a good bit of grunting and heaving and it finally broke loose!