finally got around to modifying mine.
i got in touch with vintage speed and asked how they would recommend modifying it. they said just bend it.
carefully wrapped in cotton and tried to bend (combination of vice holding it secure and pipe bender. shaft material looks exactly the same as the stock one. it shattered again. it's clearly solid steel, but jsut really poor quality steel (although plenty good for a shifter.)
anyway - worked out very well. i just cut it off with a saw where i wanted it, chamfered the end after wrapping it in tape to protect the paint
cut off the threads on the shift nob side and chamfered those threads too.
adjusted my welder setting to be as 'tig-like' as possible (high power, low feed)
welded it on - while keeping it cool (to not wreck the paint)
filed the spot welds down and bobs-your-uncle. done.
while i was in there, i took it apart and loosened it up a bit and lubed it up with grease. (shifter movelment and reverse trigger)
very happy with it now.
(BTW anyone who installs a kafer bar will need to adjust the shifter back ~5mm.)