Torquing an aluminum plate won't create the same stresses as a cast iron plate will, and if your using a cast iron head, then you'll need to use a cast iron plate to reproduce the same distortion as the head will when it's torqued. But if your using an aluminum head on the GMC, the aluminum will need to be machined out of one of the high grade billet heat treated alloys, or if your casting it as it sounds, it will also need to be poured from a high grade alloy such a A356 and then heat treated.
