Start soaking the old bolts with PB Blaster, or something similar now. Repeat daily til your new manifold comes in, then spray one last liberal dose right before removing them. The machine shop that milled my head hit the bolts square with a hammer, then broke them loose with an impact wrench. None of them broke, and it had shrunk about 1/8" overall. Of course, the head being on the tailgate of my buddy's pickup helped immensely, but still not a bad idea.
Daniel