I agree with much of what is said above.
I've used a smaller hole in areas where metal was thinner. Best thing to do is take two scrap pieces and mock up what you want to do.
After a mock up of a couple different size holes and techniques now try to disassemble it. You'll find which methods and sizes work well.
Even smaller holes in thin metal are often much stronger than the metal itself...
More frequent spot welds will also increase strength.
In a few areas I have used a mig with the heat way up and used no hole and gotten great results...problem with that is a little too much time spent in any given area is a big hole thru 2 layers...
The opposite side of the panel on this type of weld will produce a good match for a factory spot weld...