My 5.9 4x4 durango with 220,000 miles will flat pick up and slap around the 4.7 2wd durango at work with 1/2 the miles and never miss a beat.

I think they are rated 1 mpg differance in the same car so you might as well go for the 5.9. Easiest way is to get a donor truck to swap parts from. Wiring harness, computer and exhaust are definately different, both use a similar motor mount but I don't know if they are exactly the same, both have 3 bolts that go straight into the side of the block, you will need the trans from the 5.9 truck because the computer for the 5.9 won't run the trans from the 4.7, you might have to swap transfer cases if you have one and you might have to swap driveshafts. If you have a same year same chassis truck to swap everything from it would really not be hard but piecing it together could be a real pain.