Install question

I will be installing my hampton gci60 pellet insert soon. In the front of my fireplace there is a air vent that goes through to the exterior of the chimney(fireplace outside air kit?). It sticks up about an inch or so. I think I can just pry it off, but underneath it there is a hole that runs the whole length of the vent 32"x2". What can I seal the with, will insulation work? Or maybe just a piece of plate steel. Also what do I do with ash dump plate? When I slide unit in will it slide the plates off the ash dump. I added a pic

The ash door I would stuff with insulation. If the doors are in the way then take them off and fill the hole with insulation. The "air kit", can you take it off and place bricks there? Is that spot needed for support under the stove? Definitely block off the air flow with some insulation. It doesn't really matter what you do, as long as you dont use anything flammable, it works and you are happy.

I've noticed you've answered all of my posts, greatly appreciated. I'm thinking I'm going to remove the raised "air kit grate" and put down a piece of plate steel. Maybe put a few dabs of high temp chaulking to hold into place. Thanks again you've been really helpful.