No such thing as casting number 6N, they are a 6H casting on a late 455 motor ,and as such they have a massive 124 CC combustion chamber!
The size of the piston dome you would need to get 9.5 compression would in and of itself a limit on power!
Try to turn up a 6X casting with a dash 4 stamping on one of the tall bosses below the valve cover on the Exh side of the head.
If you go to Wallace racing they have a diagram to show you where to look on the head.
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