I have and old style canister filter on my 29 hp Briggs and have been told by a few people that the canisters are very restricting. The guy at the shop where my motor has been for the past three months even recomended removing the filter and mounting the inlet stack on the in and running with no filter lol(and I wonder why my motor spent 3 months in the shop.) Anyway, I found a k&n with a 20 deg. Angle flange so I should be able to attach it directly to the carb without having to use any piping. My problem is, I'm not sure how much surface area I need to make sure that I'm not starving the motor of air. Maybe you guys with mikunis could tell me what should work. The filter is a straight type with a 3.5" O.D. And it comes in either 4", 5", or 6" long. I would like to get the shortest one that will work.

I wish I had the dimensions off my old k&n, but I sold the motor. I think it was 4" long, but not really sure. I don't think you can go wrong with any of the one's you've listed.

But here's a picture to help.

any of those are capable of more cfm's than your carb will ever pull.