Cylinder head question

The only problem with pressure testing heads is that they may seal cold but then the cracks open up when they get hot or at normal operating temp., this is all that they can do for aluminum other than the dye check method but this is for aviation and is rather pricey, and not many shops do this. For iron pressure testing may show a internal leak or a severe crack but then as has been stated you should be able to see the crack if it's this bad.
Most all magnums that I have seen have been cracked, and they are generally on the end cylinders as they get the hottest. Part of this is due to the thickness of the material and the foundry that did them for mopar. Thin light weight casting tend to run hotter and weigh less but they have inherent problems, and cracking is one of them.