I do believe that Ford used pan or round head slotted screws for the lower retaining fasteners. I'm using screws that are about the same as used for some of the firewall clips. My rumble seat back cushion spring assembly has the lower attach flanges that have rather large slots in them. I had to use washers on the screws to give better support. The attach flanges are riveted on to the spring structure and are made of pretty thick metal. Bolts with a hex head in a 12-24 thread might be harder to source.