I've got a set of Moss 2-point belts in my MGB, they work really well. I wouldn't go with original belts unless they have been re-webbed -- the webbing does wear out and get brittle over time and won't do much for you.
You will want to make sure you check what your mounting points look like, as different years of MGB/C had different mounts behind the seats. All MGB/C had mounts on the floor for the 2-point part, but the third point behind the seat was different for different years.