You might want to think twice about that. The shop manager at the dealership my dad worked at had a new truck. short & wide but fitted wih 3.07 gears (which was an option in those days). Did fine on the freeway, but it could barely pull a load in the bed....and don't ask it to tow anything! There was a 3.54 gear option as well. With the SM420, first gear overall reduction is literally 25:1 and it should cruise @ 50-55 mph no problem. A friend had a '64 with the heavy duty 4-speed and a Dana 60 in the back with 4.11's. He put 33-12.50's on it and it would run about 50-55 about the same as my '65 would run at 45.