You can find a shroud to fit your fan in a junk yard. it will be made of fiberglass plastic (sans cloth to reinforce the resin). It doesn't have to come off of a GM product (though I would look there first). It just has to fit under your hood and to enclose the front half of the fan. So you will need to know the fan diameter. For a GM gas engine it should be 18" in diameter. The more blades you have the quieter the fan will be and the more air it will move.
Modern air liners have curved up wing tips for the same reason your house fan has a blade shaped like the wing of a Spitfire WWII fighter plane. Air slides off the end of a fan blade or an air plane wing unless constrained by a fan shroud, a winglet or a counter aerodynamic force in the case of the Spitfire. Old four blade fan blades are stamped out of flat metal and twisted so they act like a canoe paddle when it comes to moving air (take a moment to imagine the paddle of a canoe moving through the water).
You can see the vortices coming off the paddle as you pull on it. Modern more expensive paddles are made of plastic with hollow metal shafts and have a cupped blade to capture the water.
On a five bladed clutch fan (used on A/C equipped cars) have a bent tip and an arc in the blade to capture the air instead of letting it slide off. I used a seven blade clutch fan to keep my 582 BBC cool on Central Florida streets during the summer. If you have ever been to Disney World in the summer you will know that it is a wet heat that is oppressive. Your car feels the same way and needs help staying cool.