It is a problem marqs.
It would I think be possible to machine the top part of the taper to 'parallel' and then press/shrink on a collar or sleeve which could then be turned to make a perfect continuation of the original taper. A rather fiddly job, but I'd say it was probably do-able without significantly reducing the strength of the shaft or its fit into the brass end cheek - or wrecking the fit of the ATD.
Good luck!