<font color="blue"> I think I get what you're driving at; the lip would prevent twisting with just the one bolt holding it, correct?</font>
Yes, that is why I would put the lip in.
<font color="blue">So, are you saying you wouldn't drill/tap into that cross piece?</font>
No, not at all. I don't think that it would hurt anything to drill and tap it. I just always try to use existing fasteners when I can if they prove to be in a good location.