Keith, I used a Pentax 6x7 for a little over 20 years, from the early 1970s, and it was superb. I'm moderately heavily built, with large hands, and I found its ergonomics to be excellent. Its weight never bothered me, although I appreciate that others have a different view. I bought the grip, but never used it seriously because I found the 6x7 so easy to hold by itself. Of course, you put your left hand under the body and the right one on the side, reaching over to the release. I found that particularly comfortable, hour after hour.
I used the 90mm, 105, 165 and 200mm lenses a great deal, and occasionally used the 400mm EDIF and 55mm lenses. For the first four lenses in this list I seldom employed a tripod, because I needed to work fast. In fact, the weight of the camera is an asset here--it resists sudden camera movements. Because my subjects were often moving reasonably fast I usually employed shutter speeds of 1/250 or 1/500, but even at lower speeds, down to 1/60 for slow-moving subjects, I seldom had problems with mirror slap. The mirror swings back at the same time as it goes up, and that makes the problem less serious.
However, I find it hard to envisage using the camera for street photography. It's size is only a part of the issue--the noise it makes when the mirror comes down is not what you'd call discreet!
Make sure you get the metered prism. Back in the 80s the second-hand market was saturated with used unmetered prisms. Everyone who bought one seemed to trade it in quickly on the metered variety. I found the meter very accurate, not least because its averaging mode was easy to predict. (With centre-weighted meters I tend to have difficulty figuring out what it's reading.) I always had the older 6x7 bodies and metered prisms, not the newer, matrix-patterned prisms and 67 bodies.
For a while I even used two bodies, holding 100 ASA and 200 ASA transparency film respectively. I'll agree that the weight then was significant, but it was not too bad. I could run fairly fast with a single 6x7 and a 90 to 200mm lens, but with two it was more of a challenge ;-) Nettar