Welcome to the forum @jbl375
If negtive and sensor aspect ratios don’t match, one needs to stitch or accept some loss
- The 907x digital back has an aspect ratio of about 3x4 - or 12x16.
- Your negatives have an aspect ratio of 4x5, which makes it 12x15 and without stitching, we’ll always loose 1/12 of the Mpixels at least. A 50 Mpixel sensor can yield 44 Mpixels max.
That being said, the weight of the gear requires a really stable setup, which does not necessarily come cheap. Something like this seems to be too shaky.
One way to improve stability is to attach the lens or extension tube rather than the camera body. Using some video rigging items can help as shown in this prototype.
Anyways, browsing through that thread can give you a few ideas…