I would first clean up your firewall (i.e., remove any rust) to see what you have. Take some pictures of any concerns you may have.
Would be a good time to also check your cowl for rust.
2007 Premium MustangCoupe (V6).. Please don't shun me
Replace Full floor pan
Replace Full cowl
Replace Radiator support
Shelby Drop done
Front disc brakes on
4 wheel drum to disc conversion, rebuild front suspension/steering, taillight panel, trunk floor,302 and AOD upgrade, wiring harnesses back in, and hopefully only patch panels on the quarters