vinnieschip

hello I have a problem, Im not highly knowledgeable on what this part of my truck is called, and can not find a replacement for it, because so, its (what I would call a support) for the cab it is rusted out on both sides, it goes across over the top of the frame, can anybody tell me please, im working on trying to get it street legal and need to know soon so I can get this moving , thanks.