The seat back rest spring can not be installed properly without the wood frame installed on it first. Go to this link for diagram and prices.
https://fordwood.com/roadsters1928-37.htm Also probably available from other parts suppliers such as Bratton's, Snyder's etc. Where are you located? It's a good idea to include your location on your profile. Often there may be Model A guys in your area that could help you out. As you can see from my profile, I am in eastern Canada and happen to have an extra new seat frame as well as a used spring with a seat frame attached. If you were in my area, I could help you out with the parts you need but shipping to the US would be prohibitive. Anyway, good luck and let me know if you need any further info. I just finished an off-frame restoration of my 31 Roadster that I have owned since 1959 and learned a lot in the process. Good luck!