I dunno about being interchangable, but they sure would look nice

 

yes and no. My mechanic says no, I say yes, but I haven't tried XK engine ones. I do know that the reason they are black-coated in the 4.2 is to dissipate heat, not for cosmetics.A friend hadhis polished, removing the coating, whatever the coatingis, he had to buy new ones! it didn't work, they turned blue-purple from the heat! That's why I stopped trying to outsmart Jaguar engineers a long time ago! Having said that, I wonder if the XK valve covers might work.....
Jose

 

The chrome covers from the XKE have a dish cut out in the back that sort of matches the one in the heads of the later 4.2 engines. The Series 3 covers are flat in the back. I'm sure some sort of plug could be made to work around this.

 

yes, that's the reason I'm told they don't work in the 4.2 engine. BTW, they are not chrome, they are polished aluminum, and they leak, and if overtighened, the aluminum cracks. I have them in my 3.8 engine. The original 4.2 black covers are much more resilient.