Once you have your mirrored coordinates in place, reselect them but this time you'll get both the left and right halves at the same time.
Move these to the right hand edge of the map to get them out of the way. Repeat this process with the front faces resulting in much less space being used:
We could from here scale both the front and back mirrored coordinates up to improve the use of space but we'll keep it as working space for now and do the scaling later.