MY 37 Dodge PU is on hold until I can get some other cars done, but the rear end is in. I took an easier way out and used the stock (rebuilt) leaf springs and a 68-76 Nova rear end (these cars used leaf springs) and it was just about a bolt in for my truck. As I recall it was a couple inches narrower than the 55-57 rears.

If you've measured the width (with the tires and wheels your going to use) and the 57 is the right width, you might consider using a 68-? Chevelle rear end, as if memory serves correctly, it it is the same width as the 57 unints. It also has the lugs on the top for the upper control arms.

If you need something narrower than the chevelle unit, I think the 78-up Monte Carlo units (also coil springs) are the same width as the Nova, but you'll need to check.