Brookville sells a complete floor and if some model A parts source has photos of it for the 28/29 roadster then a person can see how it's put together. The coupe models are a little different but many of the subframe parts are interchangeable. The side sill rails and seat structures are different but not by much. All the 28/29 coupe subframes are basically the same. Most of it is riveted with the 3/16" rivets.
This link is to a Business Coupe on B. Terry's site. The only major difference is in the way the top is done when compared to a Sport Coupe.
https://www.model-a-ford.com/proj_3.shtml