I don't want to tell you how to do things - and I'm a complete amateur at most all of this stuff myself -- but -- when you're slinging that bondo around - make sure you've roughed the area with some 80 grit (at least) to give the bondo a place to bite into. Slick, flat, sheet metal needs to be clean clean clean and roughed up.
Sorry -- I just noticed it (bondo on what appears to be on sheet work with no apparent prep) in these shots of the firewall work and maybe I should keep my big yap shut....