Thought they had to be a longer shocker due to the parabolics being a 2" lift,
They shouldn't really lift it much. What used to be the issue was every UK seller only sold 4 leaf pack parabolic springs. These are a HD spring and sadly will ride like ****e unless you have a couple of hundred weight in the back.
HD springs will lift the vehicle under normal conditions because they will require more weight to make them sit were they should.
Parabolics in theory should allow more flex, but this depends on the spring rate. Longer shocks will allow more droop/downward travel, same as longer shocks do on a Defender 90. The issue is, under compression the longer shock and stock location upper mount can potentially bottom out the shock, risking damage to the shock and can bend the leaf spring the wrong way causing damage to the spring.
Ideally a longer shock needs a revised upper mount. On a 90 you get away with it a bit more as once the coil spring becomes coil bound it won't compress any further, but reverse bending a leaf spring is a potential issue.
How much of a risk this is I cannot say. But I have seen pics of this situation.
As for the shocks, parabolic springs move a lot more (or should) in normal driving .This can lead to over heating of stock shocks which aren't designed for this level of continual use. A modern shock from anyone else should be used really.
On my 88 it used to have a 3 leaf rear parabolic and ProComp +2 shocks. I can't say I noticed the reverse bending of the spring, but I found a 3 leaf spring restrictive off road and too stiff.
I now run a 2 leaf rear spring and it flexes and rides much better, although it does lean a lot more than it used too or one will on a stock leaf pack.
I actually fitted much longer shocks to mine, this required modified upper spring mounts front/rear and modified rear lower mounts too.
Just to give an idea on the shocks. These are stock front/rear shocks. The Procomps where quite a bit bigger, they say +2, but I didn't measure. Sorry don't have a pic of them. The silver shock is a Rough Country shock with 14" of travel. These are what I fitted front/rear.
2 leaf para rears and revised shock mounts:
If you only plan on road use or light green laning I wouldn't worry too much, the Procomps or any other modern shock will be fine.
I wanted more flex with mine.
Flex with the 3 leaf paras and Procomps. Not bad and more than stock:
But it used to lift a rear wheel off the ground quite a lot. Here it is dangling a wheel in the air:
With my modified suspension setup it has a lot more droop, but also retained all the up travel too. It makes for a lovely stable vehicle off road that rarely lifts a wheel.
I did mod the front suspension a bit too
Full details in my build thread:
https://www.landyzone.co.uk/land-rover/200tdi-s3-88-pas-long-travel-suspension-rebuild.298104/
