Is this what they called cave and pave?

[mongocanfly]

I would get back far enough to get to the undamaged portion..or at least where it's good enough that you could hammer and dolley back to shape..
Your weld seam needs to be where you can where you can get to it to work it with a hammer and dolley as your welding...
I replaced a spare tire notch in one of my fenders,,with help from Dan and the board.!!..
What we did was do a series of tacks holding it all together..grind those leaving the spot welds a little thicker than the sheetmetal thicknes... then hammer and dolley those tacks...then keep repeating the process til its fully welded..
Keep it at zero gap between the 2 pieces..
Robert may come along and correct me .(again)...but maybe this'll help some..

[LONGHAIR]

I don't think I would try to patch that in. It's all there, it just need to be pushed around right. Access to the back side might make it tough, but good metal working is far better than filling with Bondo. Sure, you need some, but very little, just for final smoothing.
Save the patching for places where the metal it literally gone (rust)

[Woodnears]

Getting to that front corner will be difficult. I’m grabbing a stud welder today in hopes I can pull it all back out! I’m scared the metal is stretched quite a bit and I’m not sure how to shrink it with A hammer and Dolly. Any advice would be great

[rcsbls2]

I don't think its stretched, but be careful working it out. You can easily stretch it. But with access to both sides you can go slow and work it out. Look at how the paintless dent repair people work out dents, much to be learned from them. You can get that out with almost no filler!