Thank you for the comments, junksaver, swamp rat, and raggedjim!
Today I got the firewall out.
Lots of spot welds all around, then sawsall across the floor. Made sure to leave the firewall as intact as possible in case I re-use it, and only cut the braces that run under the floor, and not the little bits that extend up the firewall.
Inside
Outside
I also dug out all the bits I need to convert my welder from flux core to MIG. Going to check if the gas solenoid is already installed tomorrow (welder is at work) and get a tank.