She won't struggle in 5th or 6th gear all the time because you won't see 7th or 8th gear. She will spend her life at 3000+ rpm. I'd take a rough guess of maybe 10mpg average. A 6 inch lift and 35's will work a HEMI, but they have more power than they need...
Admittedly, I'm assuming you'll also be running all terrains or mud terrains since that seems like the direction you're going. Those don't like to roll so you'll fight that tread pattern. Plus, they will probably weigh 15-20 lbs more per tire, so you gotta get that rolling. Then you might have some fat rims with a custom offset in mind. Those will fight you as well. The lift with destroy any aerodynamics you have. The wheels and tire will not only create rolling resistance but weight the truck has to get moving and maintain.
Sorry I don't have anything positive to tell you, but this is my 2nd Pentastar. They are more than capable engines, but they make all their power up high in the RPM band and they don't have buckets of power to spare like the HEMI.