I'm not sure from the dimensions if you can run the fireplace chimney behind the oven, and still get a 30" cooking surface. If you can, I would be tempted to build a traditional round 30" oven on top. That gives you just enough space for pizza and baking.
Because the vent of the oven is in the front, you can cantilever that and any landing area and still work with your footprint.
Here is a sample photos -- though I can't tell how that did the oven vent. Is it pulled back to meet the fireplace vent, or did they skip the all together?
Pizza Oven Photo San Louis Obispo
In terms of insulation, I would recommend buying FB Blanket (or another ceramic blanket) to be safe. The oven will get hot and burn traditional insulation. Been there; done that. I tried a low-end insulating blanket in Italy (Rock wool) that I am guessing was designed for fireplaces, and my oven scorched it to where it was black, smoking and smelled up a storm.
James