Putting a Napoleon Wood stove Insert into an existing masonry fireplace with a 1980 era heatilator box. The box has three 3" tubes . Looks like removing the middle tube will allow space to run a 6" stainless steel pipe down the chimney flue and connect to the wood stove insert. The chimney has existing clay lining and everything is in good shape. The heatilator box is feed with cool air from two bottom openings and releases into the room over the fireplace via the three pipes. Looks like the throat with one of the pipes removed will be just about 6" and a touch more. Any tips or surprises to look out for?