The x1d has a 44x30mm sensor, while a 6x7 negative is 70x55mm. So if you use a 1:1 lens, at it’s closest (1:1) focus distance, you will not be able to get the entire negative. Sure, a lot closer, but not a true 1:1.
For me, shooting medium format is all about DOF and falloff from big lenses as well as the resolution. The DOF and falloff certainly come out when I scan on a crop sensor, you just loose out on resolution. That said I do think it’s pretty clear, even from a “low-res” crop sensor scan of a MF negative, how much information is lurking in there.
I’ve done a few 6x6 stitched scans which are quite satisfying but considering the time involved, and the mechanics of getting each part of the 1:1 images flat and in focus, the juice doesn’t seem quite worth the squeeze.
The speed of scanning with a DSLR is great, but the larger the negative, the more a flatbed scanner seems to be the right tool.