So here's a shot of a vert piece that I shaved a wee bit more than necessary. Notice there's more space for the weld to sit in for the two adjoining pieces. Any more than this is a bit risky.
I've attached two pics of how these mounts fit into the bumper. The interior left side's mount (facing the bumper from the back) is welded correctly. The right interior side is an example of what NOT to do.