Mine is the OE 327 sbc block with earlier camelbacks, 18 1/4" at the head faces, not including gaskets or measuring to the header flange.
I'm not so sure a more stock set of iron heads would or could be any narrower but you make a good point. Most would have gone aftermarket aluminum before all that.
Besides that, I thought the smaller 600 series boxess were supposed to be a clearance advantage? WTH?!
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