Not sure if I'd add another full set of vertical firebrick. It might bring your heat too close to the air tubes in top of the boiler.
Not to mention you might have trouble filling it without it running out the door.
I have though of adding a horizontal row on top of the existing firebrick to raise my level be 4", but I'm getting more than 12 hrs between fills. I'd caution trying to get it too deep in the firebox.