If I were looking to put together a performance oriented build(as opposed to needing to replace some cracked original magnums on my pick up)......and one of the heads I was considering was the Speedmaster Magnum replacement head, I’d spend the extra $16/pr and use their aluminum LA heads instead.
The reason I ask is I have a Magnum powered truck using the stock electronics and I've been looking for a decent set of heads that will support a 408 and let me retain some sort of a longer runner intake with factory fuel injection.