diff flip set back???

[bark]

Hi I noticed that on diff flip kits they set the axle back an inch, is that to center the wheel in the opening or to keep the drive shaft length correct? reason I ask is I am just using a set of perches of another diff and have to build a new drive shaft cause of my engine/trans swap, but want the wheel centered, I have no box on the truck so am not sure where the diff is supposed to sit?????
I did a quick search but did not find a direct answer.

thanks