Dave,
Why do you want/need to separate the manifolds?
On the Chevy manifolds assembly, the two upper-inner bolts have nuts that usually come off OK. The two lower outer bolts, the ones you are concerned with, are the ones that are likely to break.
I am in the process of separating a 235 manifolds assembly. I have the carburetor studs easily removed, the two upper nuts easily removed, but the two lower bolts worry me. I am repeatedly squirting them with penetrating oil and repeatedly heating them with a propane torch (I do not have a hotter torch).
Be careful, and good luck.
