When I replaced both my floor panels I actually used a smaller version of the rivet myself. When you cut the floor loose the support where the bolt goes through wants to rise up. Since it welds to the edge of the panel on the transmission side which is now gone. I actually put a temporary bolt into the frame to bring the support bracket back level to allow me to weld everything up. I then drilled a hole through the new panel and removed the bolt so I can pull the cab.
I plugged this hole with a 7/8' chrome push plug. You can see it in my pictures. I understand why a person would do it since some
floors will actually hump up a bit in this area depending on the level of rust out around the support.
http://community.webshots.com/user/RugerWSM?vhost=community 
