First thing I would do with that chuck, bearing in mind it is on a MT, is to set it up appropriately to alleviate damage to the quill, so the chuck is facing upwards, jaws fully retracted, and using a brass packing piece in between, take the aforementioned 4lb lump hammer, and give it one full force crack with the hammer to drive the chuck home in the quill.
I kid you not, that may apply enough force to just crack the taper free, so you can use a MT extractor.
I would also try a taper extractor, and as suggested above, machine it so it has a shallower taper.
If that doesn't work, what I would try then is to make two identical wedges, and machine them thin enough so you can apply one from either side, grease them well, and if you have a big enough vice, don't hit the wedges, but squeeze them in the vice, so you apply pressure rather than force, and it is evenly from either side.
I have got some MT shanks apart that have also been together for years, you just have to work out non destructive ways to do it, whilst applying the most effective force.