Why so much Z?I suggest you draw your spindle with a tool in place and then consider the manner in which the motor body will limit access in terms of both depth and width.That exercise will give you a more useful idea of the work that can be accomplished and you may be encouraged to reduce the Z axis,and consequent loads, by an amount.This automatically gives a stiffer machine.You may need a few more posts before you can post an image of your proposed machine and it will be useful to see it.