Trying to get a chimney liner to work.

I have an older Blaze King insert I'm trying to put a chimney liner on. I'm having a hard time getting an adapter from the insert to the liner. The outlet of the insert is 4"x20" coming out the top. My chimney inside measures at 6 1/2"x10 3/4". I know I'll need a custom piece built, but I don't know what size liner I should be trying to get. The outlet of the insert is 80 square inches but the current chimney is close to 70 square inches. Is this a big issue? This setup has been used for decades by the previous homeowner. I've read to take an inch off of the chimney measurements to allow for insulation around the outside of the liner. If I do this, I could get an oval liner that would be down to 42 sq. in. Also, I'm looking at roughly 14' for the length of the liner. Any advice is appreciated.