You're in over your head... and that's the best way to learn to swim.
"Anything that was built can be rebuilt." --my best friend.
The best advice I can give you? Don't be afraid of learning. I had barely used a welder, much less done any body work, when I decided I could change the roof panel on my Challenger by myself. I did exactly that, wiithout so much as help lifting it out of the enormous box. I test-fit the thing 13 times by lowering and raising it with two ratchet straps slung across the garage-door rails (single door). None of my body-guy friends say they could've done it any better.
That gave me the courage to replace the floors and a frame rail in my Valiant. We did that in a friend's garage because he wanted to learn about it too. The car wasn't worth saving, but I did anyhow. We ended up rebuilding a rocker, patching areas the new pans didn't cover, and fabricating a section of a different frame rail while we were at it. "Cheap tuition" is my term for it. After those two projects, I'm (perhaps foolishly) confident... I don't think anything's too far gone at this point. The friend I quoted above is restoring a '68 Charger that's even worse than your car. His is not only a 318/column auto, but triple green... but it was his first car, which I found after we'd lost track of it nearly 20 years prior. The car was so rusty, a torsion bar literally fell out on the tow home, taking half the transmission crossmember with it.
First thing: Get yourself a factory body service manual. Check and correct your dimensions, get the car stabilized (wood blocks, welded-in angle, concrete blocks, whatever) and formulate a plan, starting at the frame rails. Don't cut off anything you're not replacing immediately--there's no shame in tack-welding something to a panel that's going away later, even if that weld is in the wrong place on the old panel. You just need to hold things in place for the time being; welds can be cut and ground easily enough.
Unless you have a very versatile spot welder, most of your former spot welds will become plug welds. Do yourself a huge favor and get yourself a pneumatic hole punch. You'll wonder how you've lived without one. I have an Astro ONYX 608ST, but anything similar will do. The Astro is cheap and mine has literally punched thousands of holes without issue, as well as stamping dozens of feet of flanges (the opposite jaw is a panel flanger).
It isn't always going to go your way. Know when to walk away. Trying to work through frustration on a project like this can lead to an avalanche of failure. One small misstep in one spot can leave you several inches off four panels later. You can come back when your mind is more clear, with a refreshed attitude, and accomplish more in less time.
Try and do something, no matter how little, every chance you get. Even if you've only got 20 minutes to drill a few spot welds (a Blair Rota-Broach is your best friend, run slo-o-o-owly) you've accomplished something and kept your momentum going. The world is full of garages with cars in them that someone walked away from for a week and hasn't gotten back to a decade later.
My crappy old Valiant still has bondo, body panel rust, no appearance improvements and no collector value (I love a good beater, and even better sleeper). That car is dimensionally straight and rock solid, and I get a real feeling of accomplishment knowing I made that happen and kept it from the crusher.
You're a long way from the goal line, my friend. Be patient, keep your eye on the prize, and stay motivated by staying busy. In a year or two you'll have the ultimate "built, not bought" story and more pride in your car than just about anyone else with a collector car. No one can criticize you for whatever changes you make, options you add, or anything else--because they wouldn't have saved it in the first place.
We may not be there to physically assist, but you've got a great internet support team (and cheering section) here. Now get after it!
