Most installations in Bugeyes I have seen bolt the belts through the floor immediately behind the seat rails, as close to the the corners of the floor/rear wall as possible. This would be for standard lap belts. If you are considering something more than that (such as multi-point racing belts), then you may also need to connect into the tranny tunnel. My car had four-point racing belts installed when I bought it, but I replaced them with standard two-point lap belts, since I am not racing it.
Tim C.