I would stary out of the junk yards, junk is all you will fine. There are many companies out there that make replacement panel s to truck and car bodies. They pretty much know the problem areas and make patch panels that you can purchase and weld in. Some areas come in several sizes so you can get one that is right for your rusted out area. JC Withney has them in there catalog, but I would do a search for "Body repair panels" or truck replacement panels". I have seen several different places that sell just such panels.