I have a Dodge d350

Replacing the gears is a MUCH bigger job. It takes an experienced mechanic to set the backlash, bearing preload, pinion depth, and contact pattern to prevent gear noise and excessive wear. You likely won't gain anything anyway. A 1970 Road Runner with 4:10 gears gets the same fuel mileage as one with a 3:55. When I developed gear noise in my '72 Dart, I removed the dinky 7 1/4" rear end and went to the much better 8 3/4". Went from 2:72 to 3:23. Driving 120 miles to college each week the gas mileage stayed exactly the same but it was a lot more fun passing people when necessary. With a higher gear ratio you don't have to push so hard on the gas pedal to get going and to maintain speed. That's why you use less energy when pedaling a bicycle uphill in a lower gear. Just because your feet are going at an amazing speed doesn't mean you will get as tired as if you had used a higher speed.