Ease of installation your best bet is a 360 but it wont do you much good for #2 on your list.
The best V8 for fuel economy will be a fuel injected SBC or SBF most likely. I love the LSx series of chevy motors and regularly get 20mpg+ out of my 5.3L.
On the other hand, the LS series motors arent that cheap just yet. and its a lot more work to make them fit than a 360 would be....
Honestly, the straight 6 is a great motor when well maintained and with a few minor upgrades and I still consider it to be the best motor jeep ever used in the CJ/YJ/TJ's. It has great low end torque and will do 70 on the freeway with the right gearing, etc. Jeep werent designed to do burnouts and produce gutbusting HP and TQ... Just something to keep in mind.
1985 CJ7 Project
Phase One :
5.3L Vortec/4L60E, Moser axles for AMC20, Twin Stick'd Dana300 - Completed
Bedliner, Rollcage, Seats, BESRK Rear Bumper w/ tire swing, Soft Top, and a few other goodies - Mostly Complete
Waggy 44 front, Scout D44 Rear, 4.56's, ARB's (F/R) SOA with YJ Springs, 35x12.50x15 BFG MT - Not Even Close to beginning
Also a mostly rust-free 1970 Scout 800A That I'll get to start working on someday...
Chrome won't get you home...BUT Rattle Can Black may just get you back"