Fireplaces, Heating Stoves, Flues and Chimneys - How to decrease size of opening before tiling?

We are planning to install a gas insert soon, but the shape of our current brick fireplace opening is about 6 inches taller than the face/front we have selected to go with the insert. While we could just pay extra for custom dimensions on the front piece, I'm afraid it will not look good to just add on the extra height while keeping the doors and "logs" and such the same size. Since we're not too thrilled with the existing brick anyway, I figured a better option might be to tile over the brick in a way that makes the actual opening smaller, thereby solving both problems. How exactly do you go about decreasing the height of the opening? It looks like there is currently a piece of iron that runs across the very top of the opening, supporting the brick above. I assume we need to install some sort of similar thing??? Or can we use some other sort of metal framing? We won't be burning wood fires in the box, as we'll have the insert. Whatever we do does not need to be pretty but does need to provide us with a good working surface, as we'll be tiling over it with 12" slate tiles. Oh, and I know we'll need a permit for the insert, but do you think we'll need one for the modifications too? Thanks for any help you can provide. Our local mason is booked way to far out, so it's up to us!

What is the demension of the "infill"?