A properly sized and fitting shroud helps air movement immensely. The fact that the truck didn't have one doesn't mean the car shouldn't have one. The cowl hood does help reduce underhood temps as it provides an easy path for underhood air passing over the radiator to escape. Because of that, it may also allow for more air to pass over the radiator at speed. Go drive the car - even revving the engine puts very little additional heat into the radiator. The engine has to work to make lots of heat - and it's not working hard at all sitting still. Let us know what happens when you drive it.