It is not a question of boost, it is horsepower and tune. There are many factors that go into the equation. There is no simple answer. Better to ask a different question like: How much hp do I want to make? Then what do I need to make that? If your setup is not reving over 5500-6000 you will likely have a longer life. The rule of thumb is somewhere around 450 to 500 hp you start to enter a danger zone of blocks cracking. However, there are many examples of higher hp where things are fine (see Moovs for example). Most likely thing to happen is to blow headgaskets on 302s. Gotta have a good tune.
97 Laser Red GT-248A Wingless model, Built Lincoln Mark VIII motor-Procharger D1SC, 3 core IC
2012 Black GT-Saddle, 3.73s, Brembos, Shaker1000