I think what silyavski is saying is that the centre hole on a nema 23 needs to be about 40mm dia to clear the 38mm raised boss on the end of the motor. If you cut long slots for the motor mounting then as these are about 47mm apart and about 6mm across then they get very close/ too close to the centre hole. On a servo motor this overlap sound like it is worse (don't own servos myself)
If you need a long adjustment slot (more than +- 10mm) a workaround could be to lightly pocket the 40mm centre clearance instead of a complete cut out ? Away from computer so can't check this for sure. Is there another way?