Kris,
With the vacuum shift it looks like a 15,000# Chevrolet rear which will have 6.40/8.72 gear ratios. I don't think there are any options for swapping gears in this rear.
The 16,000# optional Eaton rear would have an electric shift motor and might have a better gear selection.
The gear ratios will be stamped on the front of the rear center pumpkin, it might require some scraping (passenger side of the face) and a wire brush to read the numbers.
Eaton only stamps the High ratio number on their 2-speed rears [i.e. 6.40], the low ratio is found by multiplying that number by 1.36 [which will get you real close].
Mike B
