The 670s are considered a little small for todays pump gas, for the most part. You'd have to be a heck of a tuner and have exactly the right cam to make them work without fear of detonation. Or you could have your pistons dished to get to a more pump gas freindly compression ratio. Mid-70s 6Xs should be fine for a 400, or a 455 for that matter.
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67 LeMans vert w/462
68 GTO bone stock
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