Robert, first of all, if you haven't already, take those bricks out of the oven right now. That is WAY too much weight for an oven shelf. Full sized firebricks should never be used in a home oven- if you feel absolutely compelled to use firebrick, go with a single layer of the 1.125" thick 'splits.'
Even firebrick splits are a sketchy endeavor for pizzamaking. Firebrick is a pretty poor conductor and takes forever to pre-heat. You also, to hit Neapolitan bake times, have to hit at least 850 degrees.
Your chances of making Neapolitan pizza in a home oven are about as good as winning the lottery. Okay, I'm exaggerating a bit here, but luck plays a role. Before you go shopping for other hearth materials, it's critical to see what your oven can do.
The first thing you need to do is see if your broiler will stay on indefinitely. Take either one firebrick or your thin stone and position it 3" from the broiler. Close the door, turn the broiler on, see how long the broiler stays on for, and, once it shuts off, measure the top and bottom of the stone with your IR thermometer.
The next thing you want to do is dig out your oven manual and get the wattage for the broiler OR take a photo of the broiling element and compare it to something like this:
http://www.pizzamaking.com/forum/index.php/topic,16227.msg167250.html#msg167250This is pretty much the holy grail of home oven broilers. Do you see the number of passes the coils make and the thickness of the coils? If your lucky enough to have an oven like this, then there's hope for you.
Even with a broiler element of this caliber, if it does shut off with the thermostat, you're most likely going to need to start thinking about mods and about how far you want to push the oven's temperature. Marlon (Bakeshack) pushes his oven to the high 700s, which is better than some of the cleaning cycle hacks you come across, but I generally try to dissuade people from going that high. If you've got a broiler like this, I think you can dial back the temps a bit by positioning the stone a little bit closer and possibly by using a slightly more conductive hearth material- definitely not steel- but a kiln shelf that's somewhere between the cordierite Marlon is using and steel.
And that's if you've got a kickass broiler. If your element arrangement is sparser/lower wattage, then, in theory, you could go really extreme with the mods (900+), but, in that situation, I would probably recommend just building an LBE.
