Stock wheels or aftermarket? Wheel width will come into play as to what you can run. With the stock wheels, anything wider than a 215 is pushing the safety limits. If you have wider aftermarket wheels, then there's more available choices.
For a smaller diameter tire, which will increase engine rpm at any given speed, I wouldn't suggest anything smaller than a 235/70/16. The 50 series tire you mention is not rated for truck use, it's meant for compact sporty cars. They will not last long under a truck due to the overloading.
