to me the earlier blocks are too much of a crap shoot for finding a thick block for the money some guys want for them,, I bought two 1975 530 blocks from two different people and both sonic'd out excellent(over 280 non thrust and over 300 thrust) and as far as the cast iron, my machinist told me that my block was as hard of material as he has ever tried to machine ,said it was much harder than any chevy or ford block.