B body rear,

Putting a B Body rear into an A body and the factory perches have already been removed when I got it. So my question is, to get the proper measurement for the perches, can I use the inside flange of the housing to place the new perches ? I had thought of removing the 8/14 rear and placing the 8/34 rear on the springs, but not sure where to take a measurement from to be sure the 8/34 rear is in the right location. It could be very possible that the 8/34 could not be centered under the car, which what I mean is one side be further in or out that the other. If the housing still had the factory perches I would be able to mark and remove them and install the new ones from the reference point of the old ones. Maybe I'm giving this to much thought, LOL, Thanks..:D