I will echo 4dabird's comment. "Take out as little as possible." My opinion is that you should NOT change out the entire firewall. That's great you bought one...you needed it, and you can now cut it apart and use it for patches. It is FAR easier to install patches than it is to do an entire firewall. None of your dimensions will move, bracing can be less complicated, and your repair will be just as good as a full replacement, and you'll maintain much of the original sheetmetal.
Based on your photo, it appears that 80%+ of your firewall is rust-free. For this reason, I see no reason to replace more than the 20% of it that is rotted...which is probably just the lower ends near the subframe mounts.
Just my opinion. When it comes to large structural componant replacment, sometimes less is more...
I feel the same way about the floor. Cut out patches of rotted metal, use smart techniques for butt-welding nice and smooth if you're concoursing it...or just lap weld and seamseal for a driver...and you can leave the rest of the non-rusted original floor in place. Patching is FAR FAR easier to fit than complete panels are...particularly when you're dealing with resto parts that are FAR from perfect...as you'll soon find out.
