Can't wait to hear how it turns out. I have a 2015 F350 DRW 2wd with the 6.2 and 3.73 rear end. We use it to pull our fifth wheel and our boat. The fifth wheel is an Open Range Light with a GVWR of 10,000lbs. I get about 8mpg's pulling it. We are headed on a 8hr trip this coming weekend, and I'm going to do my best to hold the speed down to 65mph to see if the mpg's improve. On a recent trip back from the beach with my boat I got 5.5 mpg's for the first half of the trip. Boat weighs close to the same as the unloaded fifth wheel.
I also have a 2017 Ram 2500 with the 6.4 Hemi and 4.10 rear end. I use it to pull my work equipment(up to about 16,000lbs or so). I have been averaging 12.8 around town pulling/unloaded mix. It got 16.4 on a trip to St. Augustine unloaded when I first bought it. This truck seems to have a little more power than the Ford, and I assumed it is because of the rear end.
Having said all that, I really like both trucks. I never thought I'd buy a Ram, but really like it. I also have been a life long ford diesel buyer, but only have one diesel left that I will be selling soon.
Very interested to see how it turns out when you swap the rear end.