Get hughes smallest cam, clean up the ports with a good valve job run thinnest head gaskets you can find (I like to buy a single mr gasket MLS and use one outer layer for each side with some copper spray sealant for a very nice compression and quench bump). Ford 1997-2003 5.4 yellow injectors atomize the fuel better and flow a tiny bit more fuel and can be had dirt cheap at the junkyard.

On the other hand a 5.9 bolts right in with no other changes needed butt MPG will suffer a little bit.

The changes I listed will help MPG but could also be done to a 5.9 on a stand on the side while driving you durango till the motor is ready.