The fire won't harm the firebrick, but depending on how spread out the fire is, the bricks may not heat evenly. You could have hot spots.
If the ash pan is aluminum, it'll melt at 1200 degrees. There are pictures earlier in this thread of melted aluminum pizza pans that were used as fire deflectors in the direct flame.
I thought you were going to cut a steel plate to go under your firebrick. If you have the 1/4 inch steel, then an air gap, then your firebrick, you should get more even heating. But that's just my theory. That's how I'm building my LBE, but I haven't got it completed yet.
Today I drilled four holes in my kettle and put in two half-inch diameter rebar supports for my 1/4 inch plate. I put my 1/4 inch plate on top of the rebar, then put my charcoal grate on top of the plate to make an air gap, then put my pizza stone on top of that. I just need to work on the lid vent and ceiling mods, and I'll be ready to test.
For now, my pizza stone is an el-cheapo from Target or Marshall's, and I'm sure it won't be suitable in the long run. But I can get started with it. When it breaks, I'll replace it with firebrick splits or cordierite kiln shelving.