you could try to hold the flat part (on the end of the morse taper , may have to grint it flatter) and try to unscrew the chuck. take a note that, as i posted elsewhere,that a screw may be epoxied into the morsetaper holding the chuck in place so it doesn't come off by accident so heat may be required for removal of screw. also take note of the other advice to clean up any burrs as that may be the cause of it not running true before taking any drastic action like grinding and/or welding.