Congrats on getting an SS.
Regardless what Tag will tell you, '99 SS's are the best!
I own one as well.
Your SS has an LS1. As for the "SLP pkg.", from what I remember, the only performance upgrade was the 'Performance Exhaust' which increased HP from 320 to 327.
You can see my sig. for some of the mods I've performed, and when dynoed by SLP, my HP was 370.