All replacement springs are 25% “unrated ie stiffer. If the originals are still at the standard height keep,the, as the ride is much more supple. I have replaced my shocks with Bilstein gas units and very happy with the, big improvement in handling as the old ones were the original factory ones which had done 190,000 miles so,well overdue for replacement.
The workshop manual has the full spec for springs for all Range Rover Classic including decoding the coloured stripes on them and what that translates to I. Spec