The scoops on my engine in the '41 pickup just have a screen at the opening only. Basically a bug catcher. I only travel where the air is clean so I don't need filters....LOL
They sell small air filters to fit these "frog mouth" scoops. I think you have to buy the scoops and filters as a package as the filters will not fit other scoops.
Unless you are building a racing engine, I suggest progressive linkage on a 3 carb setup.