The Fentons they sell for the GMCs are actually cast for the Chevys. The only difference is where the hole is drilled to mount to the cylinder head. I say hole because there is only one per manifold. You may need to grind a bit of material off the front side too in order to get those GMC washers to have a surface to grab.
I am not trying to bash the Fenton headers, I am only trying to pass on the fact that they are not a direct bolt on replacement. It may take a couple of afternoons to get them tweaked just right. In all a very rewarding little project.
P.S. Don't forget about plumbing the manifold heat!
