The car looks very good! I know you'll enjoy driving it. Plus seeing others admire it, too!
On painting the firewall, unless there was a color change and it isn't "white", repainting might be an option. BUT if the firewall is "white", then I'd suggest a thorough cleaning rather than a repaint. It should be body color and without the constant exposure to UV light and such, you might discover it's still pretty good paint back there. Not unusual for some undercoat overspray on the firewall, either.
You can probably find a repro CertiCard from one of the Mopar restoration vendors. Main thing would be that it's for your model year. Plus for a "Dodge" if they were red rather than the blue Chrysler/Plymouth versions. Get a Dymo Label Maker and put your name and such in there as if you were the first owner, if desired.
Congrats on getting to the point you have!
Enjoy!
CBODY67