would like an engine that has more HP and gets better MPG
Lets examine this roughly self-contradictory statement.
Horsepower, or power of any sort, needs to be generated by burning fuel. The more power you want to make, the more fuel you need. The more fuel you use while in motion (since ostensibly your power in this situation will be used as motivation for your truck), the lower your MPG.
Now, in certain cases, a higher POTENTIAL HP engine will net better economy since it doesn't have to use as much of it's potential to produce the same power, thus somewhat lowering it's fuel needs.
Tuning and the design of the engine factor into this as well. A 5.0 is obviously larger than a 4.6 and most people equate that with more power. The 4.6 however has a better power to weight ratio and gets better mileage. Better tuning, lighter engine, and such cause that.
Within the realm of easily done, more power and less fuel consumption is not going to happen for your truck. Frankly, the 4.0 is plenty of power for your truck. If you are trying to pull things that you can't, well your brakes aren't gonna stop that load, and your suspension won't handle it.
If you want to go faster, you don't need to. If you are unsatisfied with your overall "feel", maybe you are in the wrong vehicle. As was said to me earlier this week "It's a Ford, not a Ferrari".
If you haven't tuned up lately, do that. Filters, fluids, spark plugs, wires, coil pack, and a top-engine cleaner. If you have and you still aren't getting what you want, the most easily done option for you, go 5.0.