I agree 60K is preposterous. However, 40K might be possible. I think you are underestimating the amount of people that would attend such an event. It wouldn't just be UND/NDSU alumni attending. It would generate a lot of interest from any sports fan with North Dakota roots. It would never be the Texas/OU shootout but it could turn into something special.
IMO, most fans can't wait for this rivalry to get started again. I have only been to one football game since I graduated and that was the NC game in Alabama where we lost to GVSU. I would most definitely make the trip for the Sioux/Bison game no matter where it is played.