I'm in total agreement with Terry(69firebirdman) and Tom. Terry could not have said it better. I do my own bodywork and after years of practice, I can turn out decent work. ALL of the aftermarket sheet metal is thinner than factory. Any sheet metal you replace should either be cut off a donor car or found NOS. IF, after searching diligently, you can't find factory sheet metal, then Goodmark is OK. The corners are not as crisp as factory and the metal is thinner and easier to dent. I have dented a Goodmark door sanding! In my opinion, GM sheet metal is the way to go.
Jim