Hey Chris,
I have slammed a few dubs and my experience is that you will end up doing it more than once if you dont have the full weight (engine and tranny) in it, even moking it up by bolting in the tranny and engine will give you a more realistic idea of how low the car will sit.
there is a tool available for lowering the rear torsion bars, but I usually do it the backyard (dangerous) way with prybars and jacks the result will be the same
Also measure your fender lip to the ground before taking it apart, I always notice sag on the driver side, this sag will still be there when its lowered, ulsess you install rear adjusters or new torsion bars.
When you are done and bolting the spring plate in, remember where your rear alighnment was before you tore it down as it is adjustable
hope this is useful for your beer party
Bry