I like the granny low 4 speeds.
To keep originality and hauling usable, you can change rear end gears... I have had a 76 dually with 4.10 gears and didn't like driving above 60 mph as that was when gas mileage really went down. Was a 1 ton crew cab with a 454, TH400 auto and it did great as a truck.
Best bet I would think, look into add on overdrive unit behind the trans.
Great looking truck, I love heavy duty
Bob