The thing is... Depending on if your useing a 4 link or what ever,, You need to know what your going to have in front of the rear end because this will say where your crossmember will be, then work back from the crossmember,, You can tie the crossmember into the front part of the frame,, Then set you 4 link or what ever your useing off it, Make the frame kick up's, tie back into your body in the back by the body mounts,
Really hard to give a step by step on the web,, But the car will need to be set up at ride height so you can figure everything out...
Once you have the tires under the car in place,, You can take your measurement from inside rim to inside rim.. This will be your over all measurement for the rearend,,(minus the thickness of your two hubs..)
Where people get into trouble is not setting the wheels being use under the car (IN PLACE) before starting anything.. By doing it this way the tires will end up where they need to be.. Doing a bunch of figuring without doing it this way will have your tires sitting somewheres under the car,, Might end up not being where they should be.. It's really easy to do,, When it's done the right way..Do what it takes to have the wheels right.. Are you will be sorry later when it's to late..