I had the same set-up you are describing on my Model A when I first built it. 1972 era 355 sbc (no road draft tube), intake mounted oil filler with breather, no breathers in the valve covers (corvette aluminum finned covers). I ended up using a cotton filter on the breather due to blow by, to keep oil drops from splattering on the body.
Worked great until after about 300 miles, the first time I ran the rpm's up to about 6,000. It blew the seals out of the fuel pump and puked oil all over the header. I thought for sure I blew up the engine, based on all the smoke I saw in the rear view mirror.
I rebuilt the fuel pump (Holley) and then installed a moon breather in the driver's side valve cover. I then installed a '67 Corvette oil filler tube (with a pvc valve port) in the manifold, installed a pcv valve, and used a cap instead of a breather on the filler tube.
I haven't had any issues since, no more drippy oil blow by issues, no more blown fuel pump gaskets.... it's been troubble free for the last 9 years.
Hope my experience is helpfull to ya!