Hi all, I'm interested in putting a Regency Ci2700 fireplace insert in to my existing masonry fireplace. Had the installers come out to take a look first and it turns out my flue opening is ~4" and evidently they need ~6" for the chimney liner. What are my options? Another vendor said that you can "chip out" some of the bricks to accommodate the chimney liner. Is this advisable? Thanks!