I don't know enough about MF rangefinders to offer any suggestions there. But I also re-read the OP and don't see any suggestion that it has to be a non-SLR. Several folks have suggested 645 cameras but the OP clearly says 6x6 or 6x7. Of the cameras already suggested I would think the Mamiya RB is the most difficult to handhold. I have one and definitely consider it a tripod camera. The Pentax 6x7 is hand holdable, especially with the broomhandle grip and focus levers but still it is pretty heavy and clunky. I think one very good way to avoid camera shake is to select something with a lens shutter rather than a focal plane shutter. Yes, the RB is a lens shutter system but as I just mentioned, the body is monstrous. My suggestion would be Zenza Bronica. The SQ-A is 6x6. The GS-1 is 6x7. The interesting thing is that the GS-1 is actually lighter and thus easier to hand hold. With the speed grip and AE finder it feels and works like a 35mm SLR, albeit an overgrown one... If you hunt around a bit you may be able to get one with the grip, AE finder and normal lens for $400.