jim haseltine
Senior Retro Guru
I wonder if the top nut and race have been backed against each other with such force that the spacer has acted like a crush washer in a hydraulic union. Maybe teaching grandmothers to suck eggs here but my technique for tight headsets is to get the spanners on with the free end of the top spanner an inch or so to the left of the end of the lower spanner then squeeze the two ends together like I'm using a pair of pliers, usually either the top nut or the race will move enough to free things up. Helps to put a shim of some sort like a headset spacer between the spanners at the headset end to try to stop them sliding off the flats.