Fred: Going from the 5.14:1 rear gears that are in there now down to a 4.11 rear gear ('67-'72 GM 3/4 ton w/automatic and coil springs) will yield a 25.06% improvement...not just 20%. It's significant for sure.
I am getting 15 mpg on my 3/4 ton truck pretty consistently and would think that you should be right in there with everything properly balanced. My '54 one ton is a HydraMatic so it's a different animal entirely.
I think your mileage expectations are realistic and very attainable. Obviously, the duals on the back end are going to cut into your mileage some. Bucking up the tire size will help but how important is mileage to you? You can play with a number of factors as suggested above but as you make those final tweaks for mileage you begin to compromise performance.
5.14 would be the standard rear end on that truck out of the factory.
