Considering how bad it was I can't say how much of the original sheet metal is left on the car, can't be much, and the fact it now has A/C I assume the cowl and fire wall was just left as one piece when replaced?
Actually, he's installing "stealth" AC rather than factory-style heater box, etc. He's using a non-AC heater box, some components from one of the aftermarket AC vendors, and custom-made lines that literally circumnavigate the
exterior of the engine bay. That keeps as little AC equipment visible as possible, since the receiver/dryer is on the outer core support by the condenser.
The only lines really visible on the assembled car will be those off the compressor, which disappear under the battery tray.
As such, he's got part of the Coronet's AC firewall with a large chunk of the Charger's non-AC firewall sectioned into it. The ends of the cowl are Charger, the center (and visible) area are Coronet. Which leads us to Bob's comment...
I drilled and removed a cowl for my A12. It was a nightmare getting that thing off without a sawzall but I made it all the way out to the A pillar before I gave up. Doing the firewall at the same time doesn't seem like it would make it any easier.
Apparently it's much easier with the roof off, or rather removing the roof as part of the process... but obviously it's still an enormous pain in the ass. There's a lot of overlap/layers in that area. He fought with it for awhile, and though the details are lost to the fog of time, I remember thinking "I would've been throwing tools halfway through this." It was one of the more-difficult tasks, since there was some tomfoolery with the A-pillars and windshield frame, which are from the original Charger as is the entire roof structure and quite a few other two-door-specific parts (the Coronet donor was a 4-door sedan).
Just one more process to integrate into absorbing the scale of this effort.
For the record, the engine in the pictures does not run. It was literally for getting the AC parts designed, routed, fabricated, and installed. Everything shown below was assembled and installed to facilitate only that, after which it was completely removed and disassembled prior to hanging any exterior panels.