340, count the factory spot welds.
You need as many stud welds as factory and preferably in or very near the same place as factory welds. (for proper collapse and crumpling of the structure in case of an impact), God forbid!
5/16 hole is my call.
Also, I don't usually give this tip to anybody, but for you....
Since the duration of welding a stud weld is mere second(s), the workpiece may not be ready to accept a sudden and huge temperature change to produce a weld puddle that can melt all 3 pcs. involved in the weld, therefore producing a week and brittle fusing between the mig wire and its surrounding area. In the cases where a good fusing is required but the work piece or the ambient temperature is too low (its cold outside) it is recommended to slightly warm up the work piece,
(a propane torch is more than enough, even a heat gun)Just so the surface of the metal is not cold to the touch.
In a sheetmetal application, you don't want so much heat that it would distort the metal, just slightly warm to the touch not HOT.
You'll be amazed how something so little could make such a difference.
Also, the area of the weld has to look shiny,
that is clean!
Good luck