As far as the cast cranks from PMD and failure in my own experiences, I've never run any crank in my Pontiacs other than a cast crank. So the only thing I've seen break one is when a rod breaks off, and the flailing rod gets between the block, and the rod throw. That includes from a 326, to a 400, which all use the same basic 3.75 stroke crank. I've also run a slew of the large journal cranks in racing applications, 428, and 455 cranks have not failed, unless an errant rod gets in their way.
I believe that the engineering on the Pontiac cast cranks was so over engineered that the sheer mass used in their construction made them far stronger than any other major manufactures cast iron cranks. Smokey Yunick eluded to this over engineering when he appealed to the engineers to reduce the size of the main journals of the 421, years later after his statement it's been pretty well proven fact that the cranks will still survive without the excessive overlap from the main journals, and the rod journals. The engineers had a formula that a certain percentage of overlap was need to assure longevity of the cast cranks.
Anyway I've run cast cranks in small main journal engines as well as large main journal engines for season, after season, on a dirt track without failures. Some were prepped, and magnafluxed, some were run just as the factory sent them out the door. The only Pontiac cranks that I've seen break under just normal driving were the 301 cranks, that cut too much mass out of the cranks it made them weak. The same engineers that engineered the large overlap on the 421 cranks would have been spinning in their graves when the 301 crank appeared.........
Anyway those are my experiences over more than 50 years of wrenching on them, and racing them. I hope that my experiences can give you some background on the information you're seeking.
One other thing I'm aware of in the cast cranks that GM used pertaining to the harmonic balancers. On the GM/Detroit Diesel 6.2 and 6.5 engines, which also use cast cranks. If the harmonic balancer fails (common problem) and you don't catch it quickly, or just ignore it, the cranks will fail, and break pretty quickly. Of course gas engines, and diesel engines have much different harmonics acting on their cranks.
GM would have been money ahead had they choosen the Pontiac V8 to convert into a diesel, rather than the olds gas engines, The early olds 5.7s diesels had crank snapping problems when they first debuted in 77, and had to re-engineer their cast cranks to withstand the diesel torsional twist.