I have 2 amps and a crossover mounted in the spare tire well since I didn't get the tire or jack with the car. I also have a 3rd amp about the size you have sitting on the shelf opposite side of the trunk. For a double-DIN, I'm pretty sure you'll need to buy a firebird/camaro specific install kit or do some cutting to the dash to fit a universal kit. Factory radios were NOT double-DIN, they were a little shorter.
Do not cut holes in the firewall to run your power wire, and ground at the trunk. There is a good ground point near the trunk latch behind the carpet.
Look for rubber grommets plugging existing holes in the firewall and go thru one of those. I used the one behind the ECM/PCM (passenger side near firewall) It's up high and protected so slicing a little X in it to feed the 4ga wire thru was no big deal. All wires (power/RCA/amp remote) can be ran behind the front passenger kick-panel, along the door sill, behind the rear sail panel, up to then under the trunk panels and then into either the spare tire well or the shelf depending on the side you choose. If you choose to run down the drivers side, have the power wire cross *inside* the dash, not in the engine compartment.
Once the door sill plate is removed, its simple to run wires under the carpet that it holds down. You may have to pull the bottom rear seat to thread the wires up into the hatch area, but that's simple enough. Very easy to run wires in these cars.
Do use a harness adapter for the new head unit, do not hack into the factory harness. Only connect the power wires from the harness adapter to the head unit, and Instead of wiring the head units speaker wires to the adapter harness, tie the wires from the amplifier into that harness. that will save you running wires to each location.