I'm sure quality varies from year-to-year and from manufacturer to manufacturer, however I just looked a '66, C-10 stepside with front fiberglass fenders. I inspected them very closely because I had never seen them installed on a truck, although I have seen them on display. I was not impressed with the fit or finish. It appeared they had flexed at some point and there was some evidence of stress cracks at the mounting hole locations. Perhaps there are high quality glass fenders and other replacements on the market, but nothing beats the OEM sheetmetal, I you can locate it for your year.
