If you could get a better picture that would be great. I have a 66 that I stripped down to do metal repair. When I removed my fenders the white piece you are referring to came off as part of the fender, but there are several bolts all along the inside of the fender and some of them are fairly tough to find and remove, especially if your truck has a lot of gunk or rubber-coating on the inside like mine did. The first thing I would do is try to clean up the fender and look for bolts you might have missed.
