A bit more info: it's a 300 6 cyl with a Carter YF carb. I have salvaged an air cleaner but it has no hoses attached. The PCV valve is now connected to the vacuum tree below the carb on the intake manifold, though I'm unsure this is correct. The oil filler cap has an open port. Should I just connect this to the port on the underside of the breather?
The PCV valve takes a manifold vacuum source - so the way you've got it should be fine. However, I will try and make it out to the shop and look at how mine was set up (I had a Carter on my six before I switched to a 4-barrel). Most carburetors have a spot specifically for the PCV valve - so I'll see if this was the case for mine. Please send me a PM if you don't hear back from me.
Is the PCV valve in a grommet at the opposite end of the valve cover? If so, then the open port on the oil cap is the intake for the PCV system - it needs a breather filter to filter the air being drawin in through the crankcase by the PCV source. The factory air cleaner should have a provision for a breather filter, to which a hose connects to the oil filler cap. You can also use an aftermarket oil cap that has an integrated breather filter (I did this when I switched to an open air cleaner).