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I have a 91 F350 dually with the 460, and even with the optional 4.10:1 axle, mph loaded is not the same as empty. It has an MSD 6A ignition that made no measurable improvement, a cam which did, and the 460 has never h ad any heads that didnt have excessive cross sectional area in the intake ports near the intake manifold. Experimenting with epoxy is about the best hope for getting a 460 to rival a peanut- port 454.
Mine has the E4OD, and has seen 14 MPG empty. One spike to 15, never repeated, yet. I bet the 3.55:1 gears would help, not sure lowering would prove worthwhile, but since mine has the early Sterling 10.25 axle rather than the Dana 80, a 3.08:1 gear is a real possibility.
There is a 3.23 for the D80, but the 3.31 is far cheaper.
If you have a non-dually, a D71 from a late 80s 2wd Cummins offers a 3.07 gear, you would have to swap the entire axle assembly, but it is stronger than the 3.08:1 10.25" Sterling.
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