IMO
Don't plug it, use it the vent the crankcase. I can't find my data but I think I used an 1/2 male to 8an similar (NOT) this link.
https://motionraceworks.com/products/push-in-fitting-for-factory-gm-ls-valve-cover-grommet-motion-raceworks-34-008ls
Go fitting to a catch can, pull on the...