Along the lines of what mitzel said about the stock air cleaner, that means that it has to be intact too. No holes drilled into the housing for more air flow like a lot of people did back then, or an open element filter setup.
It all has to be sealed, and connected to the cold-air intake port that was standard on all '73 and later EB's, as well as the hot-air riser off the manifold or header tube.
Even the pre-73's, back when they still checked those, you couldn't have holes drilled or an open element, even though it still took hot air from under the hood.
They wanted to make sure your air was hot from the moment it started, to reduce whatever emissions were prevalent while warming up on the choke.
So, while you can probably get away with an aftermarket or non-Bronco air filter assembly, it has to be a sealed unit, pulling cooler air from the outside.
Paul