Some background, bought car in 2004ish, it's been a Michigan car it's entire life and as such is rusty as heck.
What's been done:
-Driver floor pan replaced from kick panel to rear well
-Gas tank mounts rebuilt
-Front window frame rebuilt and windshield actually attached to body
-Rebuilt motor and transmission
Went to pull the motor the other night and found the passenger shock tower attached via hopes and dreams. Spoke with scottrodfab and ended up ordering a repair section, mostly because I'm too stupid to declare defeat after dragging this thing around for a bit over a decade. Tomorrow I'm pulling the motor to see what I'm up against.

Wow you got some work to do there! Where in Mi are you?
I'm north of Detroit, Great Lakes crossing area.
Got the motor pulled tonight with my 6 year olds help, had the fuel feed line hang up but it came loose with some persuasion. Separated the trains/bell and got the old motor supported off to the side.
Got a better gander at the frame rust, and it's a bit bigger than my patch piece from scottrod. The inner tower pieces will be an invaluable time saver though on the fabrication side. Current plan is to cut somewhere halfway up the kickup and rebuild the rail until the rear stabilizer bar bracket

How in the hell did the car not fall apart?
Like he said.......hopes and dreams. Hey, if Jerry can do it, you can too. :D
It's really a feature, think about how much work it is to drop the cradle. This body has optimized its geometry to the point where the cradle can be removed from either side of the frame rail.
I bought a TC parts car years ago that was rusted so bad in that spot the hood was only thing holding the car up! I popped the hood and the strut pushed the hood up and broke the safety catch on the hood and the whole car dropped 4"! And I drove it 5miles to my house. lol
That whole area is kind of a piss poor design. All the gaps let water in and it just sits there. When the area is repaired it makes sense to just weld all the seams solid, so it's harder for water to intrude.
Found a bit of rust on the driver side rail as well, but it's small time compared to the passenger side. I do think I'm going to just seam weld it all together.
Went through the s bin at work and pulled a workable pile of 12/16ga sheet steel, should be enough to make a solid first pass. Little bummed I moved the car into the middle bay in my garage, I can't reach the car with my tig welding leads so may need to borrow a small mig for the bulk of this.
Dug this out of my pile of books, too bad PB sucks. Would love to share this in full resolution.
I've been using Flikr with good results so far. If you have and old Yahoo account, just use that to sign in. Just have to be slightly crafty with the codes.
I actually sped a TC with rust half as bad as yours in the same area. You're a bad bitch to even attempt trying to get that back together.
Made some decent progress today, all the brake/fuel line clips came loose.
-Pulled wheel well liners
-Got the calipers pulled and hung
-Dropped swaybar off the frame hangers
-Broke the rear K member bolts loose
-Broke the strut bolts loose
Tried to drop the driver side front Kmember bolts, promptly broke the tabs off the nut plates. Got them turning with a breaker bar, but decided to let them soak in oil for a while and try it later. Or borrow a nut splitter.
Passenger side I'm just going to cut through them via my engine bay access hole.
The work isn't particularly difficult, once the engine is out. If you have a welder and the skills it's something that can be tackled at home. Now paying someone to do it is where it gets $$$, due to the labor costs.
Do the mustang floor pans fit these or do I need to just get metal and patch them in?
Zach you will need to patch them, the mustang floors are different. With that said, it's not a particularly difficult pan to create from sheetmetal if your existing floor is there to take good measurements from.
Isn't that always the case ;)
Managed to get the driver side Kmember bolts out, it was a battle. Pretty happy i can just cut the passenger side, cause that was zero fun.
On the bright side the bolts are still pretty much the same diameter as when new. I've seen some very rusty K-member bolts that were only half or less of their original thickness.
I'm blown away by what LMR and 50resto charge for these crossmember bolts and j nuts, I understand they're special, but 200$ for four bolts and nuts?
Looks like I'm getting some used ones haha.
Jebus. Yeah just eBay it. I've seen all 6 K-member bolts from a Fox Mustang listed for $50-100.
A grade 8 or better hardware store bolt wouldn't cost more then $15... you car cutting and replacing the entire area. I wouldn't be worried with originality...
One word; "Fastenal".
Check these videos out, this guy kinda shows what to do on frame rails and shock towers. Not a write up or how to, just showing you what you are getting into.
https://youtu.be/CykDm_pen 15syI
Been there bud. I sandwiched a 2x3 .083 piece of box tubing in what was left of my frame rail and built off of it. Its all a challenge but I think I rebuilt my frame rails and strut towers for under $50 hardware included
Did some work today, removed the spot welds and pulled the sway bar bracket. Going to bend up a U channel sometime this weekend and see how it fits in there.
These might be helpful, but they aren't cheap:
https://lmr.com/item/LRS-FRRH880/1979-93-Mustang-RH-Oe-Style-Frame-Rail
https://lmr.com/item/LRS-FRLH780/1979-93-Mustang-LH-Oe-Style-Frame-Rail