The factory manifolds are quite adequate. They are similar other than casting number to what came on a 69' 300 HP 350. There is a 69' Chevelle with an L48 300 HP 350/TH350 combo that has run in the Pure Stock Drags, his best is in the high 12s. The rules of the class are very strict. If you are pleased with how your car sounds and performs, sandblast the manifolds and apply some Eastwood coating to them.
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1972 SS 454 Chevelle LS5/M21/12 bolt 3.73 posi
1969 Malibu COPO L72/M20/12 bolt 3.31 posi
1968 Malibu LQ9/4L80E/12 bolt 3.73 posi
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