1965 Valiant slant-6 225 carburator

Great googly-moogly…someone sawed off the top of the carburetor. Dass eff dup.

I think you're looking at a new carburetor, fer sher. I don't necessarily think an intact one will be too tall for the hood to close. Get in contact with some of the guys over on slantsix.org who have done this swap and ask them about it. Get in touch with the 2100-expert I linked and ask him for some dimensions (max height...) then make a cardboard template that same height and see how much hood clearance you have. Note that the so-called "performance" air cleaners (open-sides type) often have taller installed heights than the real air cleaner you'd want to run, that is the OE type with thermostatic snorkel.

K&N's pretend air "filters": Not if you like your engine. It's one thing to run K&N filters on a race/competition vehicle that gets an engine rebuild every season. It's quite another to do so on an engine you actually want to keep for awhile. K&Ns will keep out spiders, seashells, small children, and chunks of concrete. Actual dust and grit flies right through 'em and into your engine. See here and here. ("…After only 24 minutes the K&N had accumulated 221gms of dirt but passed 7.0gms. Compared to the AC, the K&N plugged up nearly 3 times faster, passed 18 times more dirt and captured 37% less dirt…"). That's objective science right there. The idea that real (i.e., pleated paper) air filters are inherently restrictive is a myth fabricated by the K&N people to sell their filter-shaped toys.