Some water passages need to be plugged off, along with (depending on the year of the 258) sleeves to compensate for the 4.0's 1/2 head bolts compared to the 258's 7/16. The 4.0 head really doesn't do a whole lot of good unless you go with its or better cam. The 258's won't take full advantage of the heads breathing capabilities in other words. You'd have to run a 4.0 header, and make a little modification to the 258 intake to fit the new head properly. All simple stuff though.