Under-bed spare tire mounting questions

[dsy1]

I just got a '71 longbed fleetside with the mount under the bed for the spare. It didn't have a spare, so I picked up a used 16 inch rim and tire, but I'm not sure how it fits in the mount. The cross-strap has two brackets riveted onto it, but they don't seem to line up with the size of the spare (they are closer together than the inner diameter of the spare wheel). Do I grind off the rivets and move the brackets? How are the brackets supposed to fit inside the wheel rim? Pics would be great, or an explanation of how the spare is supposed to be secured in this mount.

Thanks.