being told 2 different thing about a rear end

looking for a ford rear end for my mustang and I'm reading and doing some home work and 2 different web sites say 2 different things about the width of the Granada rear end.

this site says the Granada rear is 52 1/4 wide
http://ultrastang.com/Page.asp?PageID=6

this site says it is 57 1/4
http://www.dfwmotorsport.com/Fairlane/9inchrearends.htm

And to top it off im get on craigslist and im looking for a 9 inch rear to buy and i see people saying "ford Granada 9"rear for sale 58 inch wide"

theeeeeen i see another "1960 T bird rear end. complete ford 9in rear end. 58 inches wide" well from what i was reading the 1960 T-bird rear is 52 3/4 RIGHT?

ok so what is the Granada width? 52 1/4 57 1/4 58......... my gosh. i am sorry for asking this question because it gets so many questions but when you see different answers from different pages and then some for sale adds with different measurements then web sites its like SH**.
Everyone must be measuring different ways im thinking. some are measuring rear end only some are measuring drum to drum. am i right about this.

Ok. i thank you guys very much. I was going to start getting under some cars and making my measurements but with all the different web sites with different info i was not sure what the hell was going on. I didnt want to drive 1 hour to someones house to buy a rear end just to not know what real measurements were.