Just my .02:
Cracked manifolds are pretty common in the mid to late 70's cars of all
makes. When car manufacturers started putting in EGR's, they basically just
drilled holes in the exhaust manifolds, regardless of the effect the hole
would have on thermal stress. It took a couple of years [or more] for the
manufacturers to finally get around to fixing the problems.
Anything that increases the temperature of the manifold [clogged cat,
retarded timing, etc.] doesn't help the situation, but these manifolds will
all crack eventually.