Sorry for the slow update. Have been too busy lately.
This is the ball screw I am going to use for the Z axis:
It will be mounted on the front, where the original lead screw used to be. I made a mounting block for the ball nut out of aluminium:
The mounting block will be bolted to the bottom of the saddle. And here is the mounting block I made for the bearings housing:
The bearings housing will go into the round bore and the stepper motor will be mounted on the opposite side. I will be using timing belt drive between them. The elongated bore for the motor will allow for belt tensionning.