I understand what you're saying and you can lay them tight with refractory mortar, but you cant lay a solid firebrick that tight with regular mortar. You would have to shake your regular mud so thin it wouldnt be any good and couldnt get a good looking job that way.-You should at least make the joints as thin as possible, 1/4 or so.
In this case, I have two intersecting walls with a tri-angle chimney base and a thimble and flu on the back side. Just make since to get the fireplace and chimney base done before they start framing.I have seen a lot of block foundations, but have never seen one with the firebox laid up that early. We are always under roof before they come back and lay that up.