For mounting the steppers, I cut a stack of 1/2" and 3/4" plywood squares 2 1/2" square. By testing various combinations of different thickness, I determined the right size for each stepper.
I marked the center of the square and drilled a 1.5" hole with a Forstner bit and drilled the four stepper mounting holes in each square. Glue, stack and press them together. When dry, cut a notch to allow the block to be slipped under the stepper. This is necessary because of that three-part coupling spider - you really need to get that assembled first, then slide the spacer under the stepper. The notch must be wide enough to allow the coupling to pass through.
To the left is my Z-axis stepper where you can see the stack of plywood for mounting.
To mount the motor, I used 10-32 x 3" machine screws. All I could find were round-head slotted screws, and the head diameter was a bit to big. A few seconds on the grinder and they fit fine. The screws to through the axis assembly - I pressed in four T-nuts. (for the Z-axis, the T-nuts were pressed into shallow holes bored into the plywood.