http://www.monsterslayer.com/jeep/40Head/40Head.htm
I've never done the conversion myself, but I've seen the results and it is impressive. The 258 head flows so poorly in stock form that it hinders any other attempts to make horsepower with that engine. With the superior flow of the 4.0 head, the 4.2 can really be brought to life without the expense of extensive port modifications and flow-bench testing of the factory head.