Installing a liner for stove through rectagular opening

[acesneights1]

I want to install my stove in our new house and the opening where the damper was is not 6” so I need to get the liner past this opening. I know they make a 5’ section of oval to round liner but how would I go back to round ? My Jotul has a round opening.it is a single story ranch and I want to sit the stove in front of the existing fireplace and go up the existing flue.

 

[acesneights1]

Sorry the pic is so bad. It has a pillow stuffed innthe hole right now where a pellet stove I think once went ou. The damper is missing. They had made block off plates

 

[bholler]

Sorry the pic is so bad. It has a pillow stuffed innthe hole right now where a pellet stove I think once went ou. The damper is missing. They had made block off plates

Cut it out so you can go through the damper with round