Re: GMpart number for ZLX heads
Based on 'The Complete do-it yourself guide to top Street Performance' 1972. "The ZLX 454 Phase III...dyno rated at 575 hp. The ZLX designation indicates the use of open-chamber cast-iron heads, first introduced to the speed field a couple of years back by Rosen on his personal race car. They have since become a performance standard.", Executive Editor, Martin L. Schorr
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