I once removed the fixed cup on a '77 Gitane Gypsy Sport using the Sheldon nut method. It turned out that despite being a Gitane that model had an English threaded bottom bracket, which I was lucky enough to discover before I starting trying to force the thing loose. Even so, it took an awful lot of force. I believe I ended up using a three foot pry bar on it. Here are pics of what my "tool" looked like afterward. It made me so happy I kept it, and it's still in my tool box looking just like this.