I have an older Heat King wood burning insert. It has a rectangular flue output (approx. 22" x 3"). Would like to make or have built an adapter that would go from this rectangular shape to 6" ovalized double wall stainless steel liner-pipe. Oval would transition to 6" round. Round would be insulated and go up to top of chimney and capped there. Really don't want to do any of this, but was told not to run a fire in the fireplace because it was poorly designed (way back in the 80's, I believe) and could start a house fire... Thoughts? Adapter design considerations? Advisable at all? I'm thinking that double wall SS, insulated pipe inside existing chimney would not overheat the chimney, etc. Appreciate any input!