Steve, from a theoretical "pure function" point of view, I feel that the reproduction fitting is probably the best BECAUSE it has the smaller hole or orifice diameter.
The aluminum intake manifold, on the GT-350's, left a lot to be desired regarding the porting of the PCV. Unlike Ford's "standard" design of using a spacer plate with the PCV discharge flowing into essentially all cylinders, the GT-350 manifold is ported primarily into cylinder #4 and, to a lesser degree, cylinder #8. This design causes leaner (more air) mixtures into these cylinders than the other six cylinders. This is less than ideal and, over time, could actually cause damage.
By using the fitting with the smaller hole, there is less air flow than if the fittings with the larger holes were used. I'm fairly confident that said air flow will be adequate and could improve your idle and/or overall engine smoothness. I've played the orifice game (on other engines) to help control the amount of crank case air that is entering into the air/fuel mixture to make the carburetor idle air screws more responsive and controllable.
I hope this information helps and and isn't too confusing.
Frank
Like Jeff has stated: Overall appearance is another issue.
