Nice truck Dukes I don't see any floats on the front yet
As for the F-Superduty trucks you are SOL on the gear selection the tallest you can go is 4:63s. It does have something todo with the housing as the Dana 80 used in these trucks are rated at 11,000lbs.
My truck has 5:13s the truck is pretty much topped out at 62 mph my other F-Super I had would do 68 it was 460 gas it had 4:63s.
You wouldn't want to go any taller than 4:30s if it was possible or you will really lack the pulling power. These old F-Superduty trucks will easily gross 15,000lbs with a load on them my gasser truck grossed 18,000lbs a few times.
If your looking at getting better mpg its not going to happen you just have to not look at the pegged tach and the fuel guage needle dropping