If you put a 351 into a 2000 V6 Mustang, I imagine you should be able to tell if the rear ends swap out or not. It would take a quick glance and a tad of mechanical know-how.
Shouldnt be too hard. If you can do a motor then the rear end is a brezze. I did a rear end swap with a buddy of mine for the gears. (he had 4.10's and i had 3.73's) and it took us about 6 hours, taking our time, and scwrewin off most of time lol.. and we never done it before but had both rear ends dropped in about and hour and 1/2.
Its pretty simple but you HAVE to have someone help you.. you could do it alone, but i know that i would have ran myself into the ground.. it would be hard alone..