Just have a question on how exactly the stock air filter works on a twin cam. Maybe I'm missing it and it doesn't seal entirely, as I imagine is the case. Where does the air come in through?

 

I think the great majority of the stock HD air cleaners have an air intake at the lower leading edge of the air cleaner/backing plate. Should be easy enough to see with the cover off.
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It's easy enough to see even with the cover ON.

 

Look on the right side of the bike, the football looking cap/cover. It's under there. Behind that, there's a big flapper type looking valve. When you roll the throttle, it opens up the valve and allows air through......well, you know the rest.

Good luck.