hello from New Zealand, 18 spline head ache a833.

No way I`d try to add a piece to the shaft. I`d lop the end off up to the taper, precision drill and bore, to a press fit with a proper hardness stock a few thou. oversize. Tig weld. chuck up in a lathe, "if possible" and turn any runout to size. You or your machinist better be good.:popcorn: