The exhaust manifolds found in the 1965-1973 Chevy and GMC Big Block engine were designed to combine the exhaust gasses from each of the cylinders quickly with a manifold that was cost effective to produce.
The 4-1 Mid-Length off-road headers are designed to allow the exhaust gasses to flow more freely from each 4 cylinder bank and combine in a more efficient manner.
The Tubing has been optimized for the 6.5L (396ci), 6.6L(402ci) 7.0L(427ci) and 7.4L(454ci) engines.
Chevy 502 big block generation 6 engines with aluminum cylinder heads will have clearance problems on some applications. Bottom of header flanges may hit head bolts.