[ACK]

Having just MIG'ed a bunch of small patches on my bed, I'll offer up the following suggestions, some that you may have already contemplated. These are based on the truck being a daily driver:

1. Looks like it's a small area, don't bother ordering a patch panel, use sheet metal and bend to shape
2. If you can't get the same gauge, go a little thicker, not thinner
3. If you get thicker metal, have it flush on the interior and protruding on the bottom. Stitch from the thicker material onto the thinner material.
4. Stitch weld both sides, not just from the inside, to minimize pin holes
5. When grinding down the interior weld, grind only the weld, don't hit the existing metal since it's already thin from the rust
6. Do minimal grinding under the truck, the extra material will add strength, plus who's going to be looking under there anyways

If you weld underneath, be careful, a glob of molten metal falling on ya' will be painful.