Swapping to an 8.25" won't move the wheels. The BBP 7.25 and 8.25" rear axles are the same width. Adding the disks in the back will widen the track width a little though, usually 3/16" to 1/4".
I have read these posts, even figured my offset for grins from the chart. But still cant see why a person would care about offset, if you got the backspaceing right and the wheels/tires fit !! Who gives a flip ?