it welds at much lower temperatures. Thats why aluminum welds are so much larger - the surrounding aluminum heats up so rapidly. When you weld with less heat though, that automatically means less propensity for warping. Everything is localized. The hood on our cars is already braced with an underhood "skeleton". So structurally, you're very safe. Welding two holes the diameter of a pen will cause some very minor localized warping. Body filler that amounts to how much wasabi you'd put in your soy sauce will take care of it. In fact, if your body shop guy just did a tack weld on the underside - while it's still hot, he could practically pour liquid aluminum into the hole. A little tack welding on top before it cools down and you could grind down the hump like a wooden dowel.
