The 340 manifolds were probably designed for high flow at high rpm and wide open throttle.

The official Chrysler literature from 1992 and 1994 describe the new design exhaust manifolds as "equal flow" ports.

Since both years Magnum V8s had egr systems those model years,
i suspect Chrysler did research and testing on these Magnum manifold ports to make sure that each cylinder ended up with the same amount of egr,
since the exhaust manifold back pressure influences the "internal egr" left after the exhaust stroke.

The fact that Chrysler later made the 1994 and after outlets smaller seems to point to these Magnum manifolds equallizing the flow from each port.

When you retrofit Magnum exhaust manifolds onto earlier engines i bet the engine runs a bit smoother in typical 1500 to 3000 rpm part throttle daily driving.

The fact that at least one side of the Magnum manifolds flows very well at high rpm and wide open throttle is just an added benefit.