This isn't something I thought up. It's an old-school trick that's been done for over 50 years. It's in a lot of high-performance books for engines that have smog heads. I originally thought the same thing, but I have worked on heads that have had this work done and have never seen an issue. Also, you need to remember this is poured in at over 1300f. Then when it cools, it contracts so it would never expand more at 200f then when it was at 1300f.
Iron heads > aluminum heads when bolted to iron blocks. All iron engines have less of a likelihood of pushing head gaskets vs aluminum headed iron blocks. When I built my 408W I could not find any modern cast iron heads. For a mud truck losing 35 pounds is not an advantage when you give up cylinder sealing and head flex.
That's more of an old wise tale. Main thing is that the aluminum head has a thick deck. There are Ls engines we build 1000-1500hp iron block and aluminum head. Everything just needs to be machined correctly.
Did you preheat the heads? If you carbon soot up the areas you don’t want the molten aluminum to attach to it’s easier to remove it when you’re cutting it all out with a burr. It flakes up easier.
