A '57 Chevy pickup is a Task Force truck, not an Advance Design.
GM used the same rear axle design on 3/4- and 1-ton trucks up to I think 1972. There are a couple of ratios available that might better fit your needs, including one real hard to find set that is about 3.70 (I don't recall the exact ratio).
I think the 3.whatever is most likely found in a later truck with a big block and an automatic, but I've never found one. The swap is pretty easy and can be accomplished in only hours according to some who have done it.
