1st autocross run.

If you're not running sway bars right now, I would start with there. If you are, what sizes are on there right now? Adjusting the shocks isn't really going to effectively control your body roll, and if you go overdamped on the shocks you'll just end up with a harsh ride.

Do you know what the actual wheel rate works out to be for the 500# springs? Maybe Denny will chime in. You might also ask wracks71 what he's running for spring rates on the AlterKation set up he has on his car, he spends a good amount of time on the track. I'm running a 300 lb/in wheel rate with my 1.12" torsion bars on my Duster, and I still run a Hellwig 1.125" front bar. I haven't run the car at an autoX yet but I'm pretty happy with the handling so far.

As far as movement in the rear, that may have a little to do with the arch on the SS springs. While the lowering hanger may take care of the ride height, the more arch the spring has at ride height the more side to side movement there's going to be. Ideally the springs should sit fairly close to zero arch at ride height, as that helps to limit the side to side motion. Obviously it won't get rid of it entirely, but it does help. Also, are you running left/right SS springs or two of the same? If you're running the SS springs as intended with the left/right springs the difference in arches and spring rates from left to right will not help your cause on an autoX course. If you're looking to spend a lot of time on the autoX course I would do something different for rear springs before I went to a panhard bar.