Morris,
I blocked off the two breather holes in the spray oiler valve covers, as I hadnt researched engine breathing that much when the motor was built, and was going to plumb -12 lines from these hole, I also made up baffles, which only gave a small amount of clearance from the rockers - too messy! so my solution was to block these holes and it appears similar to your round plugs, I prefer the clean look!
Your figure of 1 3/8" breather are you referring to the ID of eg. K&N filters?
I agree with your recommendation of two breathers for the engine and basically same for the pump to
oil tank, so what I am considering is run two -12 engine breathers to the Peterson tank, one from the valley and the other from mechanical pump hole, as you recommended! The
oil tank can either have two K&N filters on the 1/2NPT ports, or two -12 lines going to a separate breather tank.
Where did you obtain the nice breather tank, or was it custom made?