I can give you a few tips:
For best results remove the box. Yes, it's a pain if you have stubborn fasteners, but the job will go much nicer, and the results better than trying to work without taking the box off.
0.035 wire is too thick, and requires more amperage, causing excessive blow through and warpage.
0.023" wire is what you want, ER70S-6, along with a shielding gas mix.
Cut the new cab corner below the crimped body line, you never want to go that high up. Also, the replacement part does not match the profile correctly in the body line.
Sealer will help keep water intrusion into the seams at bay, and aids in reducing noise. Brands of body filler and sealer matter less than the type of product.
Urethane sealer is a much nicer product than the 'rubber based' sealer. It skims fast, through cure faster, sandable, and spreads better.
Body filler: for the first coats I like to use a body filler with milled fiberglass / short strands, it is stronger and less moisture sensitive than regular body filler. After that, you can use a regular filler over top, it spreads nicer, sands better. ( have to really pay attention to the glass filled filler, when it gets hard... you are grinding if it is ugly )
Use a 'surform' file to shape the filler once it gets hard enough before sanding. You want to remove excess, but not too much before sanding.
Sanding filler: Start with P40 grit on a block, then progress to P80 when the profile is good, then P180 before priming. If you have to add more filler, repeat at least P80 if it is spread nice and you only had to fix up a small depression. Again, P180 before primer.
Feathering the paint edge: P180 grit on orbital, held flat. When the sanding disc seems to be getting dull, change it immediately. You will do yourself no favors to use dull sandpaper. Switch to P400 for 'back sanding' the feather edge, and go from the area where there is still paint down to the bare metal.
Yes, the box side can be straightened out to look like new, slight skim coat of filler needed.