Keep it in third gear!
Overdrive with 2.79 gears is for land speed record racing, it will take way more horsepower then you have to pull that gearing. My '37 Chevrolet pick up with a stock 250 (140 HP) T350 automatic and 3.08 axle will run 70 mph and get around 15 to 16 mpg depending on how hard and which direction the wind is blowing. Switching from a 3.42 to 3.08 didn't change the mileage much, but it did lower the rpm. Slowing down helps, these trucks push a bunch of air with flat windshields. I can get 19 mpg if I keep it at 55.
1958 235 Chevrolet's, net hp 125@4000rpm, net torque 195@2000rpm
http://chevy.oldcarmanualproject.com/chevyresto/58t102.htmGear it to match the torque, 3.37 or 3.91.
Joe