I've just racked up about 10K miles on my JK / LS3 conversion and thought I would pass along a few things I learned from the swap. I have a 2008 JKU Sahara and at 94K I installed a new LS3 crate engine and 6L80E transmission in it. The Jeep is completely stock otherwise but I pull an M416 trailer with it all over the country and just got tired of being passed by, well, everyone on the road. My son and I did the swap ourselves using an RPM Extreme conversion kit. I used all new LS accessory drive components and was surprised how easy the swap was using their kit. Frankly, not having a hoist available was the hardest part but we removed the body with no problems other than added time.
The swap took us two weeks of actual work and could be done in less time if a hoist and equipment to fabricate an exhaust system were available. The RPM wiring harness was the easiest part as it comes tailored to the specific engine and all leads are labeled. This is a high quality setup. The car had a six speed manual trans previously so it made a little more work for us changing to a new center console but no big deal. Previously, the car got about 21-22 MPG on trips. It now gets the same or better if you drive it right. Mileage goes down considerably above 70 miles an hour. Best I have gotten over two tank full's of gas is 26+mpg but it typically gets about 22 with little problem. Around town it gets 17-18mpg if you drive it with a light foot. When I previously crossed the Rockies I was shifting down constantly but now the car hardly shifts down at all. It just goes. The conversion kit comes standard with tap shift which is great in the mountains.
I am extremely happy with the swap because now I have the car I want with the right engine. Anyway, would be happy to share info with anyone thinking of doing the swap. I kept list of all part numbers, etc, and notes to share if anyone wants to do the swap. My advice to anyone getting ready to make the swap is to do your homework and get all the parts BEFORE you start the project. Getting the parts right and finding the best prices was the time consuming part. Try to stay with all GM / Delco parts if you can. Give John at RPM Extreme a call if you have questions too. He is extremely responsive with email and return calls and is the guy who designed the canbus link that makes the LS ECM talk to the Jeep CM. He knows what he is talking about. Have fun.