I'll second the comments about shooting more than portraits. Although the portraits and interview information will be important in themselves, you should shoot in/around the plant environment (before and after) also. This way you will have a complete body of work relative to this bit of history and can use it in multiple approaches. Once it's gone, you can't go back. Someone suggested using 35mm for everything other than the portraits; this is a good idea, as it will be much easier to handle. While I think that using LF for the portraits is a great idea, I would use whatever equipment works best for the rest of the project, so that it gets done (even, heaven forbid, digital).
Good luck with this. It's an important thing to do and, who knows, you might get a book out of it.