Hey Jason,
The '56 with improved heads and larger Manifolds were designed to run with dual enhaust.
The earlier "Nailheads" regardless of being a 264 or a 322 all had the "Famous" "Y" PIPE
In order to accomplish this, Buick designing enginners put a hole in the frame crossmembers in order to achieve easy access of the additional exhaust pipe on the drivers side, so the pipes run straight through and the '56 exhaut manifolds were designed for just that purpose along with better volume for breathing.
Regards,
Tom Gallagher
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