Welcome to the world of restoration... your story sounds a lot like mine. I originally just wanted to spray some new paint, and change the suspension to something modern. That was exactly TWO years ago! It transitioned into a full restoration. I even had the body acid dipped. But, I do have a full Global West Suspension now, with coil overs front and rear.
Any way... when it comes to fixing rust, your best bet is to cut it back until you find standard 18 gauge thickness with no cancer. Then cut, shape, and weld in a new piece. Be prepared for a ton of surface finishing... the picture below shows where I was two weeks ago, those are the original un-damaged panels. Notice how much Filler I had to apply to get them flat. The front end now has Epoxy primer. Now working on the rest of the body, which will be faster now that I know what I'm doing. Like you, I've never done this before, that includes TIG welding too... Ohhhh, and I thought I was going to do the body work and hand it over to an expert to paint. No expert will touch it, if they didn't do the body work themselves, so like Bear, I'm going to be painting it myself too.