What size breather goes on A stock 318 carb?

The base of the air filter lid? I think it's something like 2-3/8? I'm not totally sure.

However, if you want to put a 4bbl air cleaner up top the 2 bbl. , Mr. Gasket makes a plastic ring adapter. I think trans dapt also makes this item. Auto Zone had it for myself.

The pack of rings adapts the 2bbl carb neck to a standard 5 - 5-1/4 (I think it is or something like that) 4bbl air cleaner base.