Collector of "someday" cars
Hole size, for me, depends on what I am welding, ie, external panel or structural. The spot welds on GM rockers and inner structure, for example, are 3/8, so I drill larger holes for those areas. The larger diameter provides more strength from more area joined by weld.Right on! On the hole size, I actually use a 5/16"! It lets me roll around the hole with more control.
The reason I suggested 1/4" for your cowl, is it was basically for appearance, not structural. Smaller holes on appearance panels have less chance of warping, from a smaller, shorter time hot, weld process. It, also, hides better under trim, sealer, etc. without needing much, if any, dressing after.
Yesterday was somewhat of a bust. I'll load the pics and explain later.