Service manuals and parts books for the GMC engines are available in reprint editions from several vendors. Rudy Purcell, an Inliner, may still sell the "California Bill" handbook on GMC modification, which is dated but still useful. Back issues of _The 12-Port News_ offer a wealth of information, and there are some helpful articles on the "Tech Tips" section of this website.
God's Peace to you.
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