+1, get the correct spindle. It virtually impossible to achieve the necessary accuracy with the described manual process. Besides, once it was done, you could not tighten the crankarm sufficiently, as the spindle end would be protruding beyond the end of the tapered hole. You'd have to cut several millimetres off the end of the spindle and probably tap some more threads into the spindle hole. If it's a Maxy spindle style with protruding studs, it's even more work.