When I install A/C I added the shroud. It's made from 18 gauge sheet metal. I soldered the seems along with rivets to hold it together. The fan was a 18" diameter flex fan from a local swap meet. The key to a good shroud is to keep it off the core enough so the air has room to flow through, make the turn and exit by-way of the fan. This summer I replaced the 18" flex fan with a smaller solid steel, 6 blade, 17" diameter from Vintage Air the A/C company. I figured they would know about air flow, the price was right, and the spacing of the blades was 50% forward of the mount, and 50% rear of the mount, most are not split like that. I don't know if the shroud helped or not, I never had issues with A/C till last summer when it got really hot out. The smaller solid fan moves twice the air the flex fan did, it will idle all day now and not heat up. At the same time I changed fan sizes, I changed to a smaller diameter water pump pulley, this speed up the fan 175 rpm at idle speeds. I left about 1/2" to 5/8" clearance around the fan, and about 1/2 way in and out. Not sure the in and out spacing is super critical, but to far in the air won't be able to get out. If you keep the electric, move it to the top so it doesn't block the mechanical. Vintage Air didn't have a 16", next size down was 15", a 16" would have been plenty with a correct sized shroud, 15" I felt might have been to small so I went with the 17". The 17" shroud is just the same only it has straight sides and fits a littler closer to the fan blades. The fan blade is really close to the harmonic balancer bolt, maybe 1/8" or a little more! The smaller diameter water pump pulley is off a short water pump small block Chevy engine, it lines up really close to the stock 6 cylinder balance grooves.
Edit: The core is 17" across, two row aluminum. Shroud is 2" out from core. The smaller shroud ( not shown ) has the center lip bent outward, the one shown, is towards the core. I actually made a mistake and had it flipped over when starting to bend the sides. The circle cutout needs a lip to add strength, if you are a good welder, a round rod welded to the edge would also work.
