If you draw a line through all of the exhaust manifold bolts on an old SBC head, the center of the ports lines up with the center of the bolts. But on a Vortec head the line through the bolts is higher than the ports. In many cases the pre-Vortec manifold will bolt up and often it will seal but the mismatch exists and the potential for leaks and reduced flow with an older manifold is real.
There are aftermarket rams horn style manifolds that offer larger ports. There may also be the option of using a Corvette LT1 exhaust manifold. That manifold is shaped roughly like a Ram's horn but is designed for a higher HP engine than the original Ram's Horn.
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