From what I have read, it appears some people have had trouble riveting the sides on the rear seat with the C-frame because the rivets set several inches above the base of the frame. I have taken some pictures of how I did it.
Van’s recommends riveting the rear seatback sides to the rear seatback with the use of a C-frame. While some builders seem to have problems doing this, I successfully used a C-frame. The difficulty arises from the rivet head being about two inches above the work table.
As shown in the photos, our 3 1/2 inch cupped rivet set fit nicely in the C-frame and when placed on the rivet head, raised the entire setback about 3 inches off the table. I then shimmed the setback with a 2x4 and a 1x2 to level it out and riveted away.
While you can’t see if the rivet head is on the set, you can center the rivet head on the cupped set by both feel and by sighting down the C-frame set hole. I had no problem in getting the head on the set.