[Steeveedee]

The 1/2" is larger at the outlet. I had a SHP 327 in my '64 Impala, and every head that was on it fit the manifolds fine. I'd guess that BPE just wants you to run headers. I've never seen that era SBC that required a different exhaust manifold. In fact, I put a 350 Target engine in a '64 Corvette with no manifold issues. That was in '82, so that year engine.