[Shellshock]

Stick the whole thing updside down in a bench vice, with the stem in the jaws. Some WD40 down the end of the fork's steerer tube might help here. Grip the seat-tube and headtube and swing like a motherfucker. Failing that, a big soft faced mallet and a blowtorch are your friends!*

*Shellshock will in no way be deemed responsible when you set fire to your workshop or crush one of you own fingers