A 1/2 ton rear isnt heavy duty. Also a 3.73 rear end will make the engine turn more RPM s using more fuel.You are going in the wrong direction.
If you want better mileage than 15 or 16 that the chevy will give, then you need to buy a toyota 4 cylinder pickup.
If you want mileage from the chevy fix the rear thats in it, (Probably a 3.07 or 2.73) and keep the engine tuned well,and drive it easy. ,You may get 16 MPG.maybe 17 on the highway.
Fact is stranger than Fiction