I have a Vermont Castings Defiant wood stove, model #1945 . It seems I need a stainless chimney liner installed. The stove has oval shaped collar and turns into a 8" pipe and into the chimney thimble. they want to use a 6" liner. This doesn't seems quite right. can I replace the collar on the stove with another than transitions to a 6" round instead of 8". Tile lined masonry chimney aprox 8"x13" flue now. I was told that VC made another stove collar for 6" pipe use?