I can only really relay what I've been told about the reproduction Fentons. From what I was told, the headers are the same, the main difference from the originals being the rear header no longer having the heat riser cast to it, which required different castings for Chevrolet and GMC due to the length difference. I could be incorrect about that, I've never had the opportunity to compare a Chevy to GMC as far as port spacing goes.
Now, as far as the fitment and grinding mentioned, even the Chevy version requires some grinding to fit. Most of the grinding will be on the mounting tabs, as they need to ground to match whatever intake you're using. It's not really that bad to do. I ended up not only grinding the headers, but also made my own clamps out of 3/8" stainless, as the stock clamps did not really work all that well.
