#481988 = early '75 = thicker
#500557 = late '75 = thinner
And I agree with the others. That #557 block will be fine in the stated HP range. As always, spend a little extra to have the block magnafluxed. If nothing else, for piece of mind.
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1969 GTO hard top ~ Std bore 400, '70-RA3 block, 670 heads, Bal. & Blue... M22, 12-bolt w/3:55s
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