I suspect you know this already but changing a head gasket is an entirely different level of effort than replacing valve cover gaskets. I can't speak for the Jag engine, but just for the purpose of insight to what the job would entail on the 03-05 M62TU, the book procedure calls for putting the crank in TDC, removing some things, then putting the crank in a 45 degree position, removing the heads. That specific process means that you have to know what you're doing and not "wing" it. You'll want a proper service manual for the engine....I've done the valve cover gaskets before...How much worse are head gaskets? Also, if the cylinder heads are warped... does the entire engine need removed to fix?