1. Your not suppose to tamper the firewall because it's a saftey equipment.
2. Your tampering with the firewall Vin #.
3. It will fail saftey inspection due to the firewall is tampered with.
1) I cannot find any laws that say I cannot replace the firewall. If your going on safety then I cannot replace any of the bumpers or hood ( they are all part of the crumple zone)..
2) If you modify the vin# itself that it illegal, but you can cut around the vVIN and attach it to the new firewall. I have seen it done and passed inspection. After doing some more looking around in the vehicle code I cant see where you have to have the VIN on the firewall. It just states that for the years our cars were made (94-01) you just need two vin # attached to the car minimum (i.e. dash and door jam) . In a repair of the firewall the VIN would not have to be reproduced.
3) If you do a crappy job, and don't remove the compete firewall and install the new one as the factory did, yes you will fail. But repalceing the firewall is not an automatic fail