I've heard that for the 2-bolt main 400 block all '67 to mid-'75 are pretty much the same. Someone also posted saying they thought the 67 block was meatier. You would have to line up all the casting numbers and check them, or talk to someone who has built hundreds of motors and knows the ins and outs of certain castings.
For example, Dan Whitmore told me '66 blocks tend to crack in the lifter valley region and quite a few 76 455 block have some of the lifter bores drilled crooked.
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