I have done the exact type of shooting your describing, my camera of choice for this has always been the 645 Mamiya, I shoot with a leaf shutter lens that allows me sync at any speed and use a old Vivitar 285 and a strobo frame and it works great, my % of keepers is always far more than the losers.
The Pentax 6x7 is a great camera, but I find it a little heavy and un-weildly for this type of work.
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I do a quite a bit of this type of stuff with my Pentax 67. The flash sync is only 1/30 of a second but I also picked up the 90mm F2.8 Leaf Shutter Lens (only available used) and the 165mm F4 Leaf Shutter Lens for Flash Sync Up to 1/500 sec. By far for candid wides all the way to tight portraits my favorite lens is the 90mm LS lens. The only downer is that you have to "cock" or wind the Leaf Shutter every time. Also, your flash sync cord plugs directly into the lens. Go, for the 67, I say.
Ah, but the 67 can also take 220 film and the newer 67 II gets 21 frames on it. David