A flatbed truck with duals might not be a 1-ton. Assuming its not a bigger truck, during the blue chip years there was a 3/4-ton dually and you might want to be sure which this is.
The 1-ton would be a model 250, a 3/4-ton a model 150.
The standard 1-ton gears are probably the 5.14 Bobb identified. The 4.56 was standard in a 3/4-ton and both those and the harder to find 4.11 gears which were available in later years are an easy swap into your original rear end housing. Finding the pumpkin with the gears you want is much harder than performing the swap.
