rear end swap

I am also looking to swap a 8 3/4 in my 63 dart gt A body and was wondering what size the rearend needs to be, does anybody know what the width is drum face to drum face on the stock 7 1/4 that comes in these, i seen somewhere its 58 1/2 and another says 55.7 backing plate to backing plate, any help much appreciated !
Talk to Dr. Diff. He supplied the parts I needed and he is a gold mine of information on mopar rear ends.