Since you have alot of weight I would go with the tires that are wider to help spread the weight. Unless you are cruizing at 70 mph on the highway all the time the taller tires shouldn't make too much of a difference. I went from 235/70-17 highway tires to 265/70-17 all terrains and the only change in mileage was the 60+mph (which dropped like a rock to about 15-16 mpg). I know my mileage under 60 didn't drop and even improved in some cases. This included paved roads and dirt/gravel roads (still up to 60, even on dirt
).
The taller tires are going to cause a loss in power without regearing(due to a mechanical disadvantage). Increasing width will have a larger impact on mpg at highway speeds...IMO.
Figured that might be helpful since your truck looks like it sees a bit of dirt ever so often.