Read something in a tuning book a long time back. Recommended that each filter should have a capacity at least equal to one engine cylinder On that theory a flathead should have room in there for a large can of beans. Paper element on my wee 4bbl is about 6" i.d. x 4" high. Many runs on a quiet road with a gas analyser, readings all in the green mid range, so I assume thats big enough.
There is a formula for filter area needed. ci x rpm divided by 20,839 equals filter area needed. This link is for model A but principle is same. http://www.model-a.org/filtering_air_into_carb.html Divide number by number of filters. John
K&N makes an oval filter for these carbs. More filter area than the round ones. Overall, though, I don't think that much of their filters. I'm thinking of fabricating my own to use taller, larger paper filters. Get rid of those helmets.
If your going to have it on an enclosed hood I would think outside the box. I’d find a 60’s Chevrolet large round ac unit with some type of snorkel. I prefer an oval. I would fabricate a new bottom using the outer few inches for the filter fit. I’d sink it partially over the carbs. Chrome the top or even black depending or the rest of your engine compartment.
To fit under the hood with it closed Early MJ (Jeep) air horns and filters for road driving. You can put the helmet style back on at the show
Been running these for some years but I have found K&N filters (SU-7503) that are the same dimension except twice as tall @4.5" that I may try. May not be the best but it should be better at twice the surface area.