There is an episode of Four Wheeler (w/ Ian Johnson) that takes a 2.5 and swaps in the 4.0. He shows all of the work involved. It's a lot. If you really want a 4.0, then buy one. If the 2.5 is that clean (and cheap), buy it. Drop the gears down to 4.56 or 4.88 with 31 in tires and have fun!
My 4.0 with the automatic won't set any speed records either. However, the fun factor of driving a Jeep (regardless) of the motor is worth it!
My 4.0 with the automatic won't set any speed records either. However, the fun factor of driving a Jeep (regardless) of the motor is worth it!