On a 400 the 6x4 would be preferred just based on compression ratio. There really is no significant flow advantage to any of those low compression Dport heads, just chamber size differences. The 6x and 96 are considered top of the heap though. In Pete McCarthy's article "Ultimate Head" published in HPP, he found a set of 5cs that were an anomaly in terms of flow superior to the 6x.
6x8 flows better than 6x4 if you plan to install 1.77" exhaust valves due to less valve shrouding in the larger combustion chamber but you would have to mill them to reduce the chamber volume for your 400.
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Triple Black 1971 GTO
Last edited by NeighborsComplaint; 10-14-2021 at 12:10 PM.
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