Sorry, I'm not sure I understand. Do you mean I should add a keyway if I bore out the spindle, or if I use a screw in toolholder?
There is a keyway on the spindle taper at the moment. A ball bearing on the outside of the coventry chuck engages with it but it doesn't appear to be designed to take the cutting loads but rather to help you align the chuck the same way each time (I guess that the final grinding on the internal taper on the chuck may have been machined in-situ so you'd want to replace it in the same orientation each time to minimise runout)