The sheet metal on the metal structured model As is both riveted and spot welded to the subrails or body sills. The outer vertical panels are crimped to the lower outer lip of the sub rails as was previously mentioned. The subframe assembly is mostly riveted together. Some floor panels were fastened with screws.
The Model A B Ford Garage has a lot of info. This is a link to a disassembled coupe and its reassembly. A person can see how it goes together and most of the steel structured cars are similar.
http://www.fordgarage.com/pages/bodyassembly.htm