I don't know as though the solution is just because there is a washer there, I think it helps if the bolt is longer... think about bending a coathanger back and forth to break it, if you grab a long section and bend it back and forth a given amount like say 1 foot and bend it back and forth 1 inch it will take forever to break it, now if you grab it 1 inch apart and bend it back and forth about 1 inch it will break pretty quickly, that is what is happening on these hemis, they are short bolts being bent back and forth a certain amount that is much more than an iron manifold on an iron head would be bent, that iron manifold, iron head and long bolts because of the spacers is not really taxing the bolts enough to break them, they are not experiencing as much stress and not on such a short distance. I think a longer bolt and a spacer would reduce the stress and save the bolt even if it was the same material.