[DetroitDan]

I just put a sport bumper on my 82, I belive we're talking about the same thing, looks like a Blazer/Suburban bumper. It has two sheetmetal pieces on the top of it to conceal and protect the brackets from the stuff that spills out of the bed when you open the tailate. Least that's what I was told. Don't know why you'd have to protect the brackets, but I guess salt or dirt could collect there and speed up the rust. But it would probably collect on the thin metal and rust that even faster than the thicker brackets. So, the ones on mine are maybe 16-18 inches wide and about 3-4 inches deep. They are out toward the ends, and the center area is open to the elements. If you wanted to just close it in you could easily do so with a strip of sheetmetal and attach it to the holes in the top of the bumper brackets.
The guy called them pans, if that helps in searching.