The 907x/CFVII 50c comes with a thin viewfinder mask that you can fit into the V system viewfinder. Check your 907x package documentation for it ... it's in there.
I've never used it. When I was precise framing and am using a V system body, I flip to Live View and the electronic shutter. I know you like to use a strobe lighting setup so you want to use the lens shutter, but there's nothing to keep you from turning on Live View and then opening the shutter on Bulb to adjust the focus and framing, then closing the shutter, recocking the camera, and making the exposure. It just takes a few seconds to restore your exposure settings for the exposure, not a problem for the kind of still life/studio work that your lighting setup implies.
Another facet of this is that I tend to shoot with a waist level finder (or magnifying chimney) most of the time when I'm shooting with the V system bodies. The focusing screen I use fairly often has a "super slide" framing square scribed into it ... I find this scribed square fits the 33x33 cropped format of the CFVII 50c pretty closely, and that it's also pretty easy to mentally visualize the slightly more oblong 33x44 format with that in view ... it's just a 5mm addition to the framing lines on each side.
Of course, you can easily create a mask template in any simple graphics application, print it out precisely on some thin card stock or opaquefoil, cut it out with a hobby knife, and drop it in on top of the focusing screen before you slide the viewfinder you choose in place. Just create a 56x56 mm square, then a 33x44 mm rectangle, center the latter over the former, and print it out for the template. Use that as a cutting guide to make the mask. (My preference is for just a simple mask, without all the other format lines and such that docholliday does, but there's nothing wrong with either approach..).
Fun fun fun...!
G