I have worked on building such a camera by cutting away part of the film gate of a 35mm SLR. And removing the shutter. (This is sort of similar to other semi commercial projects, but my idea has some differences/simplifications because I don't have a machine shop, etc.) My project's not done but I can say a few things:
- You didn't mention a shutter, but you almost certainly need a lens in shutter, assuming the Belplasca is like other stereo cameras and has two small shutters that you'll remove. Unless you only want to take time exposures. This limits your choices.
- A lens that covers 6x6 should have enough coverage for 24x72mm.
- General challenges will be mounting the lens, focusing mount, parallelism of lens to film plane, and flatness of film. I don't know if the Belplasca is going to be particularly bad at this. It's really up to the parts that you're going to make.
- So anyway, you need a wide angle lens that covers 6x6 and is in shutter. This narrows your choices; they're mostly re-purposing wide angle lenses from some medium format system camera. Former-Soviet SLR lenses typically aren't in shutter. Inexpensive lenses from folding cameras and fixed-lens TLRs aren't wide angle. Wide angles lenses for MF were never an amateur photographer's item, so there isn't a giant supply of them lurking out there. Mamiya TLR wide lenses (55mm or 65mm) are a possible candidate. The 47mm or 65mm Super Angulon is possible, although not exactly inexpensive. It is said to possible to remove a Bronica SQ lens from its mount and put it in a Copal shutter, again not exactly cheap and also requires some work. There are some other MF system lenses and so on.