It’s not the material, I don’t think there’s a place to drill due to the shape. I could be wrong but if you look at his pic it looks like if you were to drill in a forward spot you would be weakening the arm. It doesn’t have a tab like yours do. I’d hate to see his control arm break in half going over a big bump. Inboard or outboard might be less risky but that doesn’t help him. I could be wrong but that’s what it looks like to me.that was the inez 4/6 kit on the Silverado (IHC is who makes it) the control arms are aluminum, I don't see why you couldn't drill thru steel also, cast iron might be a no-go