There are some threads on this subject. The rearend gear assembly (known as the front loading style pumpkin) from a 67 to 72 3/4 ton GM truck with automatic has a gear ratio of 4.10. These will bolt right in in place of your original. There were some Dana axels in that period which don't apply here. Your original ratio should be 4.57, so changing to 4.10 is not a big change. You can also put 235 x 85 R16 tires on to give a reasonable rolling diameter.
Some vendors like Jim Carter sell the 4.10 gear set but they can only be used in the modern gearsets mentioned before.
