Is it a good idea to install a shroud when the fan is pushing?
No, Assuming by pushing you mean electric fan on the front of the radiator, then you don't want a shroud.
For a fan on the back side of the radiator you do want a shroud.
If you feel it is a restriction to flow at high road speeds then install some rubber flaps that seal shut when the fan is drawing, and flap open to pass air when the wind is pushing faster than the fan can suck. Hard to describe, but once you see it done it's as simple as can be.. if only I could find a picture for you.. I'm not so sure it's necessary though, just a plain shroud is probably OK.
Grigg
