In general, a lower gear will help with mileage but there is a point where it could do more harm than good.
More info is needed with your specific setup- most importantly is your tire size. If you are sticking to stock-ish tire sizes and do not need to haul much weight, you should be fine with the 3.07.
What you have already should be a decent setup for mileage. Driving habits will most likely make a bigger change than gearing at this point. Above 55mph, aerodynamics start to really matter and with trucks, generally if you can stay around there, you will see the best mileage.
Can you drive around without 1st gear? That will be similar to how your driving will be with the 3.07.
Here is my favorite gear ratio calculator that you can play with-
http://www.grimmjeeper.com/gears.html
