I'm just ready to button up my '55 331 and breathers are on my to do list. Other than looks wouldn't the amount of breather volume depend on the amount of blow-by. On a fresh street-driven motor there should be little.
I'm using stock valve covers also and I hope to be able to hide my breather by plumbing the valley pan. It won't be easy fitting something under the Cragar blower manifold but I think I can make it happen.