Well first off let me say kudos to you for having the courage to step off into a project of what looks to be a sizeable undertaking I assume you don't do this for a living...neither do I but I can try to share some of my experiences in the rebuilding of my 68 GTX which required much of the same repairs as I seen in your pics. On my non-visible areas (internal structure) I removed as much of the damaged areas as I could cut out, if it couldn't be cut out I blasted to bare metal and used weld through primer then added reinforcement doublers then used POR-15 to seal it all. As I disassembled the GTX I found the same things you are seeing, rust everywhere as in it was a chain reaction type is disassembly...couldn't find a quitting place. In my case I ended up taking it apart until it was on caster wheels then started blasting, priming, and using POR-15 almost everywhere internally. I was lucky to find a donor Satellite for a lot of metal, may be a bit harder for you to find a Barracuda donor so looks like some fab is inevitable. In my case there was still lots of fab involved...it is what it is, took me 7 years and about a 2" folder full of receipts. Like I said, I'm not a professional so those guys may chime in and have more/better suggestions good luck!