Stove pipe question

[JonOfSunderland]

I'm planning on installing a wood stove in my basement and I can't find this particular scenario in any installation literature. I have a masonry chimney behind an interior concrete wall (the house and basement were added on to). There's a clay thimble going through the concrete wall to the chimney flue. The clay thimble is 8" ID and an 8" to 6" reducer fits nice and snug with the lip on the reducer keeping it flush against the wall. All literature says to extend stovepipe into the inside edge of the flue, but this always assumes there is nothing but cut away chimney/brick/wall that smoke can't safely pass through, whereas I have a clay thimble that I would think the smoke could safely pass through. If usable, do I need to cement the reducer into the wall?