Is this for an A-Body???
That POS chart is WRONG!!! 51.40 is no way in hell correct. I know two or three people cut one to that length and GOOD LUCK getting a stock set up axles in it! Too short.
The 55.6 number is the track width which has nothing to do directly with the total axle width
You want correct lengths for stuff look here.
http://www.moparts.org/Tech/Archive/axle/16.html
If the housing is about 53" spanning over the pumpkin, should be 52 5/8" linear, and perches are 43" c to c, it's an A-body rear.