The cracks arise from the head's deck height/thickness. The deck height for the cylinder head is the metal thickness of the cylinder head mating surface where it would meet the top of the block(more accurately, head gasket) in the watehr jacket area. Seems like Chrysler was looking to minimize weight, and took off a little bit too much metal.
I don't know if they are still around, but there were third party heads labelled Engine Quest, which were based on the Magnum heads, but had a thicker deck height, and no cracks developed on those. In addition to the thicker deck height, they also flowed, out of the box, about 10% better than the Magnum heads.
Almost all factory LA, Magnum, and Engine Quest heads are cast iron. I think there are aluminum heads for the LA and Magnum, but they would be third party. The Engine Quest heads originally came from either Australia or New Zealand, but I think were also later produced in the US. They were also re-sold by some well known Mopar high performance suppliers. I don't know what the current situation is.