We are purchasing a Quadrafire 4100I fireplace insert. We have a central (inside the house) brick fireplace with a metal Superior Heatform firebox (installed in 1975 when we built the house). The Heatform has 4 tubes at the top. We'll need to cut one or two of the tubes as well as some of the metal at the back of the firebox for the full-chimney-length stainless steel liner to come down through. (the chimney is currently lined with 12x12 clay tiles) Has anyone had any experience with this situation? If so, how did you create a block-off plate and where did you insert the plate? Thanks in advance for any help.