From what I've been able to determine through research, Pontiac cast its blocks and heads of GM-232-M iron. A quick google search shows it's registered as "gray iron," but I've been unable to determine its metallurgical makeup.
Two-bolt main caps were of the same as the blocks (GM-232-M) and many four-bolts caps were too. Some four-bolt caps were, however, constructed of ArmaSteel or Malleable iron, depending upon the model year and/or application.
Intake and exhaust manifolds were cast of GM-14-M iron early on and then GM-232-M later on (mid-1970s onward). A former GM foundry worker told me that 14M was softer and more easily machined and was GM's choice for components expose to exhaust gas.
We'll never know if some runs were of a special alloy or if the metallurgic makeup changed from batch to batch, but for the most part that's what I found Pontiac used. How that compares to other divisions, I don't know.
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