What is the correct width of an A BODY rear end

The CORRECT flange to flange width is 52 5/8"
drum to drum should be 57 1/8"
Perches are 43"

That chart that is floating around is a total POS.

Here's a good reference
http://www.moparts.org/Tech/Archive/axle/16.html

Yes, you can put a B body rear under an A with the right offset wheels. Spread the springs or move the perches (best), seen it done both ways.