Based on this how is it that they interchange?
Just because the head is different or the valves are different, does not mean you cannot take the entire head and swap it over.
You may have to interchange major assemblies as a whole, but they are still based on the same block and share a lot of components.
Think of a small block Chevy. Some use different size valves, pistons, cranks, etc, yet they are essentially the same engine.