I am in the process of doing this right now. However, I cheated alittle because I had the mopar MPI kit on my 4.2. I had all the wiring and intake already set up. There are a few things you need to know. First, plan on using the 4.0 accessories. ie power steering, alt, A/C, etc. The 4.2 accessories will not fit. Esp the P/S. The motor mounts and bell housing to block bolts are a perfect match. The only thing I had to do was to use a 7/16 bolt on one hole of the left mount into the block. For some reason it was tapped different. Unless you buy a bellhousing with the sensor mount on it, you will need to fab a bracket for the crank sensor. You will need to get a bronze pilot bushing for the trans of the CJ machined down to 1 inch OD in order to fit into the crank of the 4.0. There are probably some things that I am missing so if you have questions, feel free to email me. Hamy.
One Way Up Racing