I always modify the patches for fit, all patch panels that I've ever used reguardless of make model or supplier were never stamped as a perfect piece. It's up to the installer to make the part fit and properly install it. I also agree that many available are of thinner guage than original but I've also seen many that are thicker than original. I guess I'm just not in agreement with what was said: If you are considering using lower front fender patch panels for your 66 or 67 Chevelle don't bother, you will waste your time and money.
I'd take a properly patched original fender over a poorly fitting, poorly stamped, identifiable, repo fender any day of the week!