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Cast iron is not an chemical element or a chemical compound (they do not change in quality or characteristics). Cast iron is an alloy which has varying amounts of non-iron elements.

You were lucky to get 50K miles before having to rebuild a block and/or a head on the Chevrolet 216/235/261 engines. This could have been due to fuel, lubricants, filtration systems, etc, but it is still a fact that the engines did not last as long as modern engines, no matter how well you maintained them.

Even back in those times, some engines had better cast alloys. GMC engines had more nickle in the castings and were considered to be longer-lasting than Chevrolet engines (this more than made up for their homelier grills between 47-54).

There are many reasons why modern engines are better and last longer. Better fuel, better lubricants and filtration systems, better materials, and better production techniques and standards.

Also, government standards are now much stricter (not only for pollution but also for how long the anti-pollution equipment had to last) - how the companies have met these standards have also influenced increased engine life.

Another factor that has led to longer engine life is competition - in particular, the quality of Japanese engines forced other manufacturers to increase reliability and engine life.