You can run 1250 in the manifold basically all day long without much risk of hurting it. Very short blasts to 1500 wont hurt the overall life, but you dont want to run it there for too long. Aluminum starts to break down at 1450deg so you dont want to run it much past that for very long.
We had an LB7 here that I was drag racing at altitude and at the end of the quarter I was seeing 1700 deg pre-turbo. I was letting it cool down before I shut it off and it warped a head, so be careful with the temps.