Great question!! The reason that the gills are glued in the fenders other than welded is that the Mig weld is a (hard weld) and expands at a different rate then the metal all around it. This is evident with dark colors (this car will be black) when the car is parked in the hot sun (like at a car show). The mig weld will show thru the paint. so the upper part where it is in direct sunlight of the car, roof, hood ,deck lid, upper fenders and quarters I use panel adhesive. Other benifits are no warpage and the repair is waterproof . This is why I also replaced my deck lid and my hood, the glue and seam sealer on the original panels are 40 yrs old or older and is hard too. The new panels have softer glue and expand and contract correctly. Anytime you are at a car show look close at black car in the sun you will see what I am talking about.If incorrectly done you will see these problems