Re: 4.0 replacing a 258
As far as I know, Mopar Performance is the only source for a stand alone harness for the swap. The new 2000 catalog now shows the harness, crank sensor and front mounting plate, fuel pump and PCM available separately now! You should be able to use the original 258 starter, flywheel, alternator, waterpump, p.s. pump etc. if you use the Mopar front crank pickup instead of the bellhousing type off the XJ. You will have to trim off the XJ intake manifold power steering mounting bosses in some cases if your XJ engine is a '96 or newer. There is one hole on the drivers side of the block that has a different thread size than the 4.2, you will need to get a metric bolt (I believe it is 12mm, Japanese thread pitch) and open up the hole in the CJ engine bracket to fit it. It otherwise is a direct bolt in.
Hope this helps, good luck!