Yeah, I don't know of any part of an AD truck that looks like that. I'd send it back with a strongly worded letter.
Yes, the original is welded in, so you'll need to cut it off and weld the new one in place. Be careful on the location. You need to be 100% sure that the door opening dimension is maintained as original. If you have a Factory Assembly Manual, there's a dimension that locates the front of the rocker panel from the front of the dash. That magic dimension is 8.47 inches and appears on Section 1, Sheet 1.07 right near the fender mounting hole marked with a "2" in a circle.
