The changes to the 3.5L head (and gaskets) are detailed in the factory OM603 engine manual. Basically, they increased the thickness of the metal for the jackets surrounding the exhaust valves, IIRC...? There were a couple of other changes also. The newer heads (#20, and #23?) also made changes to the oil passage ahead of the #1 cylinder, and a couple of other things too. Even with the changes they're not as indestructible as the old iron head beasts, but they're much better. The original 603 head (#14) was the really weak one. They can last a loooong time if they are *never* overheated or thermally stressed, but you need to watch the temp gauge closely.
