I wonder if they chose the '92 because there were other reproductions of it (the Gárate y Anitúa "El Tigre") and wanted it to represent an 1873 Winchester? A Winchester 1873 would have made more sense although I think the receiver was longer than the '92s. Still, it the prop department had cut the barrel down to 16-18 inches it might still have worked.
I read somewhere,and I honestly don’t remember where, the 92 was chosen because it was short and light. Therefore easier for Chuck Conners and by the way John Wayne to spin it. Both used rifles with the same large loop lever. Conners also had a screw drove through the lever to catch the trigger .