I'm getting ready for the install...setting up all mods except the actual install. Will be putting in Micore and Durock under the new insert in solid masonry exterior firebox. Just occurred to me....... there is a clean out flush with floor of the existing firebox. The clean out has a cast iron plate (original) that sits flush. The clean out goes down through the existing block wall to an equally sized opening with cast plate on the vertical in the basement directly below about 4 feet above the slab. Am I good to cover the plate at the fire box with the micore and durock, or do I need to make some provisions for this hollow?