Ah do you mean how will I attach this Z-axis back plate to the X-Axis slider block?

The screw holes will be behind the 16mm Z-carriage riser blocks, so basically it would be a case of fitting the Back half of the Z-axis with motor, ball screw etc installed to the X-Axis slider block, then fitting the Z-carriage spacer block (with carriages pre-installed) to the Z-axis back plate. I will probably use 6mm DIN dowels on most mating surfaces to make alignment easier.