You must be up in the panhandle of Texas to see snow?? Yes plywood has replaced many a bed floor. Sounds like you got a nice project to fix up.
The angle strips are spot welded to the bed. Are the rest of the bed strips there to put the boards back in the bottom?? I guess if there is no rust under the fenders where they bolt to the bed you can leave them on. Up north here that is one of the places usually rusted is between the bed sides and the rear fenders.
John