2.5 to 4.0:
Any engine swap is a ton of work. If you have a 4-cylinder Jeep the inline 6 is far from a bolt in. Everything is different. You need to cut the engine mounts off the frame and re-weld them back in the correct spot for the inline 6. Then the transmission and transfer case are both different, so those need to be swapped. Exhaust is different, so plan on swapping that too. Driveshaft lengths may need to be modified as well. Then you'll still need a complete ECM and wiring harness off of a 6-cylinder Jeep of similar year. Transplanting a carbureted 6-cylinder in place of your fuel injected 4-cylinder is not an upgrade. You'll need to do something with the radiator fan and shroud. Have you thought about emissions, will you be able to get tags for your Jeep if your state has some sort of inspection? Any engine swap is a ton of work; most will agree it's easier to just buy a 6-cylinder Jeep.