Okay...I'll be the smartass....

The safest boost, is NO boost.

However, a stock LS bottom end has just over 9:1 compression. Which, with an all-aluminum combination, is low-compression. Basically, though, it depends on the turbo. If you use a small turbo, you can run greater pressures than you can with a big turbo, because the peak power levels won't be there.

What generally will happen, is you will run enough boost to cause detonation, which kills the cylinders. It literally will displace them to another location inside the motor. I've seen this referred to as "walking" the sleeves.

Safe, in general, is about 8-12 psi, depending on turbo, with a maximum of 300 horsepower. Anything over that, and your BMEP's are high enough that there is no margin for error. And, I'd highly recommend using a meth/water injection system if that is indeed your goal, as it will drastically reduce the likelihood of detonation, which will immediately displace the cylinders at that power level.