Yup, that's an "airbox" all right. I thought you were talking about the pipe-like affair that runs across the top of the intake manifold - I should have known better. Anyway, if you run that hose from the crankcase breather to the airbox, and leave the hose on the oil filler cap the way you have it, you'll need only a flame arrester in the little pipe coming out of the breather. If it doesn't have that rolled up piece of brass foil in it any longer and if you can't find one of those, you can use a piece of that pot-scrubber-sponge-like stuff they use in the oil filler cap on B18s and B16s. If you can't find that, you could use a small wad of really coarse bronzewool (marine supply) (not steelwool) or a piece of an old-fashioned bronze pot-scrubber - if they still exist. If you switch the hoses around, which is how I have them on the B20 in my PV, you'll want a PCV valve in the hose on the breather and a B18 oil filler cap which has the flame arresting bronzewool in it. I used to know how that affected circulation inside the engine and why Uncle Olaf made the change when the B20 came along, but I've forgotten :-(
Bob S.