Re: 4.2 to 4.0 head...is it worth it?
YES, do it!!
It is worth it. You will be amazed at the difference in HP and reliability. I used a '97 TJ head and MPI (with '95 throttle body and sensors)to do mine. The Mopar MPI will work with the 4.0L head, but I recommend a head in the '92-'95 time frame for use with the Mopar MPI. The head I used came with a header so I did not need to purchase one. Make sure the header doesn't have any leaks or cracks. You can do the swap with the bottom end (block) remaining in the jeep. Do a compression check (dry and wet), to help you determine the condition of the block. Once you have the 4.2L head off, you can look at the cylinder walls and check for wear (ridges) in the cylinder walls. Hesco sells all of the conversion parts and they have great customer service. I was even able to use the stock YJ air box with a little trial and error and a BFH.
The head swap is one of the best mods you can do to your jeep. Once the search is back up, you will find lots of info on how to do it.
Lots more power and 18.4 MPG city is a big benefit.
Good luck and keep us posted.