I originally had the steam vent running to the header tank as well. I wanted to tidy things up and with the change of manifold I was able to mod my water pump to put a filler neck on top. This let me delete the header tank and I ran the steam vent to a point high on the filler neck.
I now just have an overflow tank in case of a boil over. Don't want to be dumping coolant onto the track surface, tyres etc.
Ideally you want that steam vent going to somewhere as high up as possible to burp the air out of the cooling system. If you run an inline filler in the top hose just tap it into the side of it would probably be good enough.
Cheers