That kinda mileage would pay for a 6.9/7.3 non PowerStroke swap in short order. The 4.10 geared and carb'd 400Ms, 460s, Dodge 400s, and 454s were the worst guzzling motors I've ever driven esp when coupled to non OD, non lock-up automatics............but they got the same 6-9 mpgs regardless of what you are doing, and they last forever with minimal maintenance.
If she's just a driver and doesn't tow a heavily loaded trailer all the time, regearing to 3.5x or numerically lower, or an auxilliary OD unit can work if it does see big loads, can improve the gas station situation a bit.
1989 E-150 2wd Sportsmobile Penthouse with just paneling and insulation, no interior fixtures. 4.9L, C6, 3.08 R&P, 225/75-16 tires. I keep it full of construction tools. Typical daily weight is 6-7,000 lbs.
17 mpg highway, 14 city. I drive 65ish on the interstate, just get to the speed she's comfortable and hold the pedal steady mostly. Drops to 60 on most hills and hits 70 or so going down.
Expecting the M5OD swap (with 3.55 R&P) I'm working on to improve mileage a bit (20 mpg hwy is my guess) but mostly doing it for fun and to keep a 30 year old automatic transmission from ruining my day.
That E4OD you're considering with 3.54s and 245/70-16s is very very similar to my plan of an M5OD and 3.55s with my 225/75-16s. I'll turn a bit more RPM with my .80 OD vs your. 71. At 70 mph I'll be at 2,281 and you'll be at 2,004. Bet you'll be around 19 mpg hwy and 15 or so in town. If I ever get to travel the Panamerican I'll probably go to 235/85-16 for a touch more ground clearance and better all-terrain tread availability. That'll drop me to 2,105 rpm at 70 mph but that extra inch of ground clearance and higher rolling resistance will probably hurt mileage a full digit or so.