Really up to how you want to shoot. Some like wide, close but flying in all directions. Some like more distance. I've had a good time with a 40mm lens on 35m film; a 60mm is similar in 6x6, while a 50mm I tried on a Hassy was a touch too wide for my style.
Key phrase: my style. I've seen 40mm on a Hassy used well by others.
There's no right or wrong really, whatever fits your style. I use an 80mm myself, but I've seen great street work done with ultra wides, Shinya Arimoto comes to mind (http://www.flickr.com/photos/55411539@N04/). The only thing I'd avoid is tele lenses, to make good documentary style photos you need to get in close and interact with people, not snipe from afar like a paparazzo. Also, big lenses undoubtedly attracts attention and makes people uncomfortable. Not really an option with a Hassy though.
A really cool way of unobtrusive street shooting can be done with the SWC I suppose! If there wasn´t the price, I´d have bought one long ago... Otherwise I would recommend 60/3,5 or to stick with the 80mm you already have.