I found a great deal on a new Buck 91 - pre cat. My chimney is masonry, with 13 inch clay lined pipe, 22 fee in length with a 6 X 32 inch metal damper over the fireplace. The Buck 91 has an 8 inch output. Can I reduce that 8 down to 6 , so that I can use a 6 inch insulated liner in the chimney. I don't really want to take a torch to the damper, because is is heavy thick steel. Thoughts.