I have recently discovered that a 1957 plymouth uses the same springs as early B bodies and all the A bodies.

Front segment = 20"
Rear segment = 35"
Total length = 55"
I ended up ordering a pair of Caltrac monoleaf springs for my '57. I told them I wanted a pair for a 1962 dodge with a one inch drop.
The car sits exactly where it did with the pair of springs I ordered from espo springs, which I also requested with a 1" drop.
http://www.springsnthings.com/