I really think you're putting far too much emphasis on this whole "air stacking" idea.
And I'm not saying that it isn't a valid concern, it can in fact be an issue. But it's not nearly as big of an issue as you're making it out to be. Pretty much all of these cars came with shrouds from the factory. Pretty much all new cars come with them. Factory designed shrouds are not going to be that big of a concern for air packing.
The only instance where you would consider air packing as a possible issue is if you were having problems with the temperature increasing while at freeway speeds (or higher). And even then there are other considerations that would be much more likely (too small a radiator, plugged up radiator, missing hood seals, underdriven pulley, etc). The aerodynamics in most automotive situations don't really start to come into play until you've exceeded 100mph. In this instance, not only would you have to be going that fast, but you'd have to sustain it long enough for the temperature to start creeping up. And it would be a "creep", it wouldn't just suddenly shoot up. Realistically that's not an issue for very many people, especially the way these cars are used now. Drag racing isn't a long enough sustained high speed. Unless you had a VERY poorly designed shroud, you'd have to be a long track road racer for it to really be an issue.
Are there shrouds out there that could cause you issues at freeway speeds? Sure. None of the stock shrouds though. Take this one for example-
View attachment 1715496286
That's a terrible design. It's basically a cookie sheet with a couple fans bolted to it. Which is probably why they had to add the louvers. This is a Champion set up, by the way. They used to just be flat with no louvers. The shroud is too close to the core, the fans are just tacked onto the back, the whole thing is completely flat. This is the kind of thing that needs flaps and louvers to keep from air stacking at freeway speeds. Also, I'd never recommend this fan set up. There's no info on the fan cfm, and even if there was it's likely the fans won't flow what they're rated at.
Now, this is my fan and shroud set up. This is the '95-00 Ford Contour fan. It's for the same 26" radiator as shown above. At first glance it looks pretty much the same, pretty flat, fairly thin.
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But that's not the whole story. If you look at it from an angle, you see that the shroud is actually NOT flat. It's thin at the edges, but the shroud is angled back into the fans and is much deeper there.
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It might not look like much, but that's a substantial difference. The rear of the shroud is not just a flat plate, it has significantly more depth and the back being tapered into the fans makes a big difference. You can also see just looking at the picture from the back that the cage on the back of the Champion fans is much more restrictive than on the Contour set up. Those are subtle differences, but with aerodynamics the subtle differences can be very important, especially at higher speeds. I have no issues at all with my Contour fans. My car runs down the freeway in 100°+ weather at 80 mph just fine, in fact the temperature even in those conditions will trend down. If I maintain it long enough the temp ends up very close to my thermostat temperature.
How important is that efficiency? It depends. If you can keep your car cool without a shroud it might not be an issue for you. But I would always run a shroud with a mechanical fan. You can easily be losing 10-20% of the efficiency of your fan without one. Which just means the rest of your cooling system has to be overkill to make it work without one. For a drag racer that might mean you're carrying around too much weight, running a larger radiator than necessary. Or losing more HP driving your fan than you would need to with a shroud. Maybe with a shroud you get away with underdriving the pulley, and you get more horsepower back.
