There is only so much you can do without doing major surgery. If you want to keep leaf springs then you are going to need to get some decambered springs. But then you run the risk of hitting the axle case on the chassis rails if you go too low.
I dont know who or where to get some.
If you are on facebook there is a group dedicated to P100's and they are quite knowledgeable. P100 Owners.
I think we need a proper owners club and forum just for P100's.