But this is the race forum not the restoration forum. The factory probably used the same length axles to make it easy for them but the engine offset is different between A and B body cars.

If someone is building a race car and they want the engine to point directly at the pinion they'll need to move it over a small amount on an A body car compared to a B body car. All depends how serious the effort is and how concerned they are with getting things lined up correctly.