Really there's no set amount of "boost" that is safe to run on a stock block Z6. It's how much power you're making that matters, the amount of boost it takes to get there is going to very from turbo to turbo. I know alot of people that have made 250whp on a stock block Z6 reliably. I know as much as 302whp has been done, but I don't know if I would want to rely on that for a daily driver.