thats part of the lower core support. probably a piece that would have to be unbolted/welded and reinstalled. I would try massaging it back into place with a hammer next time the rad is out.
Actually I noticed there are some holes in it. Maybe try a slide hammer
1998 Mustang GT- Cammed PI Motor and bolt ons 292 whp 306 Trq, tubular front suspension, Built Rear
2010 Dodge Avenger R/T- Brilliant Black- Stock