Why not just use the 2001 truck then? What's wrong with it?
This proposed engine swap is a colossal waste of time and effort. Can it be done? Probably. But its not worth the effort. Not only are you changing engines, and transmissions, you're trying to put an OBD2 drivetrain into an OBD1 vehicle. There is quite a bit of electrical work you would have to, as far as mixing and matching wire harnesses between vehicles. Not only do you need to get the newer 3.0L motor and transmission to work in the older vehicle, but you need to make that wiring compatible with the dashboard, the fuel system, and other components of a different generation of chassis. Again, sure, anything can be made to work, but this is not even close to being worth the time and effort.
You also will not likely see the fuel savings you think you would get. Yes, the 3.0L is a smaller displacement engine. On paper, it may look more efficient. In a dyno cell, it may very well burn less fuel per hour then a 4.0L. However, the 3.0L was barely adequate for powering a Sable/Taurus. Putting it in a truck makes it substantially underpowered. That means it will have to work harder then a 4.0L to move the same truck. The net effect will be little to no fuel savings, after a lot of tedious swap work.
Just fix/replace the transmission on the '94. If you really want a project, swap to a 5spd manual. It will be more fuel efficient then an automatic.Then, be sure to keep up on tune ups,etc... on the 4.0L so it can run its best (and most efficient). PLugs, wires, as well as replacing any malfunctioning or worn out sensors (o2 sensor,etc..) can make a big difference in fuel economy.With everything properly maintained and working correctly, your 2wd '94 should be able to get in the mid 20's mpg.
There are also mods and bolt on stuff you can to to get both more power and more mpg. An underdrive crank pulley is one of the best "bang-for-the-buck" things you can do, followed by an electric fan in place of the factory mechanical clutch fan. Of course a mild performance exhaust system can also help.
