As you run that 460, that big heavy head expands and contracts, but not as fast as the relatively light manifold. As the manifold heats and cools, it gets thicker then thinner and longer then shorter. That longer and shorter, puts sideways stress (engineers call it Shear) on the bolts. So the head ends are being "worried" fore and aft, bending the bolts. The holes in the manifolds are oversized, partly to allow for some thermal shifting. But the threads as solidly into the head. That focuses the strain from all this worrying, right in the thread nearest the head surface of the port.
Great Catch, Ron! On asking when they broke. Now if they're worried AND stuck, they'll probably still break under the torque of removal effort.
