i didnt do the firewall, but everything else . i used a 3/8 spot weld cutter. cut all the spot welds, then ground off the nubs left on metal. Once you get used to doing it , its easy just takes time. I used a blair spot weld bit, used 8 bits they are double sided.
Just take your time, and use vice grips to make sure its lined up.Then screw it in place,once your absolutly sure its lined up correctly, spot weld in a few places. Then check it again to be sure everything lines up.
i started with 4 pair of vice grips at the start. By the time i was completed i had 60 pair of various sizes! lol every time i saw vicegrips at a yard sale , bought them.