Newbie chimney question

Hello and thank you, I just joined this forum and would like to thank all for the valuable help. I have been scanning some of the posts and went out and bought a 1999 Encore cat from an Amishman for $500.00. I always wanted a VC stove because I am 51 years old and remember being around people that had them and they were some of the finest American crafted stoves ever. I understand that the best flue would be 8" round. I just finished demolishing my fireplace to create a little set back room and was hoping to install the stove with a rear exit into a T and go up through the chimney with some basic 8". After getting the damper frame out I'm still looking up a narrow rectangular openig for 20".My question is would squeezing the flue into an oval shape to clear this narrowing restrict it to less than desirable? The chimney in question is sound brick 11"x12" with no terra liner. It's getting a little hard breaking bricks out up inside.