We use an oversized filter on our 07 6.0 in the Roadtrek. Wix number is 57045, but that might not be the right on for yours. I think GM did a thread change sometime it the mid 2000s from English to metric, ours is metric. They do make the same filter in the English thread, I am almost certain. I am sure Wix would be able to tell you what the number would be.
GM used the smaller filter on one of the other engines, either a 4 or six, don't recall, but they had trouble with very cold weather and were damaging them before they got oil pressure. That is when they went to the bigger one. All the specs are the same for the 57045 compared to the original filter, but it is much longer.
I have looked for a cold air intake for the Express and have not found any, anywhere. For use it would have been really nice because I have just closed up the radiator area air bypasses, and that is where some of the air for the intake comes from.
The Chevy air filter is really large, so my guess is you won't see a bunch of increase in power with the K&N and mileage won't change because of the computer control. I have used a lot of K&N in hotrods and never liked the fact that they let more dirt through. You could see it in the aircleaners after the filters a lot more than with good paper filters. The K&N do considerably better after they get fairly dirty, and most folks clean them too often.
I like a setup like they use on our John Deere lawn tractor which is made to be in a very dirty atmosphere all the time. It has a normal, high quality paper filter, but then it has a thin foam one that goes over it and is very lightly oiled. There used to be automotive ones like this, but they weren't usually oiled. The foam one catches 95%+ of the dirt, and the paper one gets the rest without getting plugged. With the tractor, we just keep a half dozen of the cheap foam filters clean and ready to go, and change them when they get plugged with the big stuff. The paper element stays very clean and there is no dirt found getting past it. I think if I were doing the dirty desert, offroad, type driving I would do similar setup for the Chevy. It would be very easy to do. We are almost always on cleaner areas, so our normal filter lasts for 30K+.