Moving rear axle back

Are you talking about that the front section of spring is quite stiff, helping to control wheel hop etc?

Right. The entire A body line got that front segment length. That's why, when you put the SS springs on a B body, you have to use the extended front spring hangers. The B body has a longer stock front segment. The front segment on the SS springs is stacked up thick as hell and short, so it acts like a factory traction bar.