Take my advice... splice the outer wheelhouse to the old one a few inches short of that joint you are talking about. I'd even leave that verticle "boxed" shape in place and have your splice on the outside of it. That joint you are talking of is like a "rolled weld" instead of spots. I don't think you can separate it without tearing up more that you fix. Besides that, there are 3-4 layers at that joint... probably why it is welded that way.