My opinion is that if you're going to be modding your car w/ a down pipe, you might as well pick up your own wide band. Innovate MTX-L is a very popular option and comes with a bung.
That said, you really don't need a fancy weld job or bung. The bung should be installed under the protection of the engine bay/firewall anyways. I welded my own with a normal wire feed. Nothing fancy is needed here.
As for the size of bung... honestly they should all be the same size. Unless you're getting motorcycle results or something(?)