i'm talking a stock block with no forged internals. how much hp before it gets into the "danger zone" of blowing up?

 

With some very safe tuning you can probably achieve 600rwhp without (hopefully) blowing anything up.

 

Forced induction or nitrous and the LS1 stock bottom end will handle 550rwhp without too much trouble. Beyond that is on borrowed time, but some have done 600rwhp for a couple years/miles before a piston ringland lets go, which is almost always the case. The block, rods, and crank are very strong.

It will handle anything a naturally aspirated build can put out, but if you want 6500+ RPM some better rod bolts are in order.

As said before, tuning is key to making the pistons last.