Sounds ok, but it looks like you didn't take out your damper, I think you will find it's only about 5in wide. What size is your flex? After you have stove and flex installed, pack it good with insulation, you want it airtight so no draft can come down chimney.
Damper door is still attached, but I disconnected the handle from the pivot point which allows it to open all the way up and back. The damper opening does measure just about 8" wide and 16"+ long. There is a Heatalator steel firebox in that fireplace, so the damper opening is larger than normal.
So any fiberglass insulation is fine?