You need accurate enough spindle control so that you can stop it within a reasonable tolerance, unless you plan on having a very wide tolerance on tapping depth.

To give you an idea of the space problem, this is a 110mm square plate which the servo motor bolts to on top of the spacer -


The spacer is high enough to get reasonable bearing spread between the two support bearings for the extension shaft, and a flexible coupler.
I have a model of the Triac head casting in Fusion360 if you want to see what will work.
IIRC you could probably get a 130 frame motor on, if you wanted a bit more power.