I'm not worried about replacing the panel, the part that scares me is blowing through the tack welds. I have Lincoln 180hd, and its turned all the way down. My settings are at A 1. I'll start Poking around and see how much metal is irreplaceable.

 

There should be no problem using that Lincoln welder. If you are having issues with blowing through the tacks then drop down to .025 wire size. I use a Lincoln 175 Plus (220 VAC) and it works just fine on thin sheet metal. You may want to double-check your polarity setup too.

 

If you are talking about blowing thru the tack welds welding in the fresh metal, you are doing it wrong. Try reading my MIG welding tutorial, and practice your tacks on some scrap the same thickness. As Charlie says use .025 wire (with shielding gas) preferably ESAB easy grind wire. Tacks are NOT beads! they should not be any larger than 1/4" and take no longer than a count of 2 to make. There should not be space between the butt joint edges. Ues a 1/2" wire stick out and a freshly clipped wire end. Put the tip of the wire directly on the seam (move it around to feel that it is exactly on the seam) and perpendicular to the surface. Hold it in contact and pull the trigger counting one, two and release, the surrounding metal should be barely glowing for only an instant. Check the back side to be sure the tack made full penetration and adjust the machine if needed. Do not make another tack within 6" until the first has air cooled enough to leave your bare hand resting on it. Adjust the seam alignment as you go, never try to tack if the edges are not aligned and touching. Have plenty of patience, welding in a patch can take several hours to days for a large patch. Have another project lined up you can work on while you are waiting for welds to cool will make the process less anxious.

 

I can vouch for this,My buddies dad started filling holes that size on his 56 and ended up with gobs 1x2 and 2x3 long.lots of warpage....

 

When you start to replace the cab corner, it would be helpful to you to have a door on the cab on that side to help you to maintain a proper gap. It is easy to have a small misalignment that is not obvious with out a door there.