Here's the opening to the chimney as I found it:
Having such an odd shape as above makes for a difficult way to seal the stove pipe to the chimney, so I stuck a 7" stove pipe in the opening, with plans to fill the void with mortar:
And here's the final product, showing the fresh mortar placed in the void surrounding the 7" pipe. Probably half the mortar that I started out with ended up on the tarp that was on the ground below. It was quite a messy job.
To seal the 6" stove pipe to the opening, I plan to use some fire resistant insulation rope that the wood stove folks provided.