How much you wanna bet that a combination of stitch welding and plug welding is stronger still? Can you test that while you are at it?
Where's the plug weld? An inch or so into the plate and at what spacing? It's an interesting idea. Punch through in a big enough shut is always a potential issue in a unibody where you're connecting to 22-20 ga sheet. You need to find a way to get it onto a larger structural member. Hard to do in the chick car.
@Mark O, I use Autocad Mechanical, Alibre and Bendtech Pro and Sheetmetal for fab instructions. I get a lot of my engineering data from Machinery's Handbook. Though even having those resources at my disposal, I look to see what the big time guys are doing. Leavitt, Blaine, Bloomquist and the like. I know I'm not an expert at this by any stretch of the imagination.