RB67 or RZ67 (RZ preferred based on my asking about the two recently myself) if, as they said, you don't want to use it handheld.
I agree about the M645, but the OP wants 6x7 or larger negative. I like my M645 Pro system quite a bit. Though I actually think I enjoy using my Yashicmat 124 more, it's much more limited - one fixed lens, no interchangeable back so no mid roll film changes, waist level finder only. Those are limitations, but also sort of help to focus ones vision. But with my M645 Pro I have the AE prism with three AE modes, motorized advance, eye level viewing that handles as well, albeit heavier, than my 35mm cameras, interchangeable backs for mid roll film changes, quick load inserts for those backs so I can pre-load more rolls if I want and reload the backs themselves quickly, interchangeable lenses (I have the 55, 80 and 150,) Polaroid back for proofing etc. (Yes the image is small but it is good for exposure verification for critical shots.) And - the Yashicmat with Luna Pro SBC, hood, filters, extra film etc. fits in a small shoulder bag made for a digisnapper. My entire M645P system occupies a large bag that weighs a ton and the camera (with prism, winder grip, back and lens) while not really bad is way bigger and heavier than the little Yashica.
There are always tradeoffs. While I've never handled one I hear that the earlier, pre-Pro M645s are quite a bit smaller and lighter, BUT they don't have interchangeable backs so no mid roll film changes.