From my own experience, using original VX self levelling springs should put the ride height at the correct level. I have fitted aftermarket 'one spring fits all' in lieu of originals previously, only to find the back end is jacked up too high. Refitting genuine springs brought the level down to what it should be. I currently have a 2.6 Elite with S/L shocks, but it has Suplex rear springs which are listed for vehicles with ride control, but it isn't right - its too high at the back.
As the Elite springs are softer and this is compensated for by the S/L shocks doing more work to maintain the ride height than standard shocks, fitting 'standard' springs which are firmer means they don't compress as much and therefore sit higher. S/L shocks are good for bringing the back end up with a load on it, but won't bring it down if the springs are too firm.
To get the same right height, a firmer spring has to be shorter in length - this applies to any car, not just Omegas.
Original VX springs are progressively wound, and the pigtail is thinner than the main body of the spring to give the 'softer ride. Many aftermarket springs have the coils of the same thickness throughout, which means they are firmer and don't compress the same under load which jacks the rear end up. This is also probably one reason why Elite springs are prone to breaking on the first coil of the pigtail where the coil is thinnest.
Others may disagree - just my experiences if it helps.