thin wall casting is a MYTH that has been debunked , the later blocks are a softer metal so that may be why mopar spread the myth in their engine book .
Also "Another issue is block weight, or the quantity of cast iron which is actually used in the block. Like the Mopar Action article, we have weighed many blocks and have found later blocks to weigh more, or have more cast iron in them. Since the reasoning behind thin wall blocks is that the factory used this practice in the late 70's to save money on iron (which is a known fact with SB Chevy engines), the fact that later blocks weigh more shows that Chrysler had no intention of using less iron to save money. Which is great news for us racers and performance enthusiasts."