I experimented with firebricks a few years ago and found them to be a mixed blessing. Very good at shielding heat, letting you get right near the coals without overlooking. But I always struggled some with enough heat as a result; they do sponge it up somewhat.
I’ve only done 2 spatchcock chickens and can’t recall how I did them, but it was probably like Jim’s picture shows, pushing baskets out like that, where you create a pocket for meat to lay in.
One charcoal basket isn’t enough heat in my experience. Either use two, or as Hoosier says, a black kettle and bank on one side. If you do that, only light enough to crest the 350-375 you’re looking for, NOT the entire bank, unless aiming for something closer to 400-450.
In other words, a big bank of coals represents a lot more charcoal than one basket, without taking up more space, but the downside is that such close heat beats the shit outta the bowl and grate straps. But in a world where cheap Craigslist black kettles are common, that might not be a practical concern. Make sense?