"Sixth Street" would be instantly understandable. No. 6 Street would sound a little strange.

I'd go with Sixth Street, if you're talking about the street's name. In terms of No. 6, you would use it when referring to your house number. For example, "Number Six George Street" is what you'd say, when you see "6 George Street". I think this would be standard in British English. In the United States, we'd say only "Six George Street". Hope this helps!

The first three posters are correct. Helen, are you wanting answers for American Eng or Brit Eng?

No. It's never done that way.