Finished laying out the floor today. Old kiln shelves for firing roof tiles. Set them in blobs of fireclay to level them out. Tomorrow, will go back in and fill all the voids with refractory castable mixed with rice hulls. This way the under floor of insulating castable is protected from wear and tear, and if a shelf cracks or breaks, it is easy to pull up and replace.
The grate bars are shelves as well, with center connection portion removed. These bars slide right into place at floor level. If one breaks, it can be easily removed and replaced. Still have about 45 of these shelves, so enough grate bars for a while.