I don't have AD trucks and Carl does. He has not said it is a factory indent. I would wait on that frame indent. I thought you had a groove and I suggested welding. I do not suggest welding that whole indent. The bracket can be welded.
Devcon epoxy is a viable solution if the company data says so. It is well known by machinists and others. Can you see any other "indents" under other brackets? The bracket does look more like a rubbed indent. I just do not see how it is possible that the frame indent has perfect radii all around a perfect trapezoid.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn