I know the 4.0 can be pretty thrifty when matched up to a manual transmission. My daughters '01 Cherokee gets 22 ish MPG cruising at 75 and isn't bad around town. The cam is small and the gears are long and it all works well together. If you want to run a carb I would guess its possible to convert. Plus the 4.0 is light, at least compared to a diesel which is an advantage off road and on. I've never done it but I understand the 4.0 head will fit a 4.2 which will give better breathing of the 4.0 plus extra torque.