Is it possible to build a masonry FP with the chimney already built in place? I have a factory built metal "heat-a-lator" type currently, but it is very rusted-through in the smoke chamber area. I have torn off the brick hearth and face already...and would very much like to build a "Rumsford" masonry FP. Can it be done w/o tearing down the chimney? (Chimney is 12" square clay flue.) Thanks.