500 is the max safely on a LS1. Not sure about the LS6 but im sure its more.
Guys are running more than that on stock bottom ends.
Kinda cross your fingers, and hope that you have a good tune.
You could get away with more than 500wrhp. Depends on your driving habits too.
If you have a LS6 with intake/exhaust + 150 shot you should be fine if its tuned to be progressive and not jet 150 all at once. Plus have all the safety features and good plugs etc....