fan shroud ?

[rallystyle]

ok I am starting to shop for a radiator for my 69 camaro with a ls swap. i see some manufacturers are using a fabricated shroud that covers most of the back side of the radiator and some are using a plastic that have the pressure relief vents. what i am really trying to find out is at what point do you need the relief vents to help with cooling. i know the suck closed when the fan turns on to pull air from the radiator core for max cooling. but when your going down the road they allow more air through the core for cooling. at what point should you have them? if your radiator does not have them. can they be added ? to me it seams that the radiator is not being fully utilized if you have to force all the air out just where the fans are. any insight on this would help before i decided on my radiator order.

Thanks