If using the stock banjo there are a few ways to go about it.
You can reverse the spring eye and lower it around an inch.
You can use longer shackles and gain and lower anywhere from an inch to......
If you use longer shackles you may need to fab a panhard bar to keep the rear end stable.
You can go with a flat rear cross member. That will gain you quite a bit of lowering.
Or any combination of the above.
Torchie.