anyone ever flow test 383 A body manifolds say vs 383-440 hp manifolds

How much difference one sees between ex manifolds and headers is directly related to what the test mule combo is.

A 9.5:1 440 with pocket ported iron heads, an RPM intake and the bigger Summit cam won’t see as much difference from the addition of headers as say a 13:1 bracket race engine with TF heads/intake and a cam that’s 272/272-106.

In addition to the power the headers themselves add, there’s also the power potential that can be tapped into with a cam selection that just isn’t happy with exhaust manifolds, but can really show nice gains(compared to the exhaust manifold friendly cam) when the headers are utilized.