I purchased a pre-fab oak fireplace surround for my black fireplace box (new construction). The fireplace is flush with the drywall. Question: the surround sits flush to the wall on the outside edge, but on the inside there is a small gap between the oak and the drywall. I assume this is to leave room to install tile/marble/brick veneer, etc. Do I install the marble first and the surround over it? Or do I slide the marble in after the surround is mounted? Thanks.