I have a 7x11" rectangular flue which I believe is 6"x10" I.D. I also have a 6" round thimble all of which service my 6" double wall stove pipe for my woodstove. I ran this for a couple months last year with decent performance but I would like to add a stainless liner for better performance and peace of mind. Can I squeeze a 6" round liner into my flue and thimble? Also, is it adventageous to insulate the liner? I have a 25'-28' tall chimney with terra-cotta flue which is insulated with rockwool between the flue and brick.