What P dude says is quite true they are nearly identical stock. Ported will be no difference either. Some pontiac "expert" tried to tell me the 6x was the best head period! Same guy thought I was nuts using a set of 1969 #46 heads too. Truth is most D ports can be ported to the same level of performance. I have a set of 6x-8's and a set of 4x-7's. Both sets stock and virtually identical maybe I'll use them some day if I need lower compression.
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