The Flathead was designed to run on fuel with high levels of lead, but even with leaded fuel, the stock valves will eventually hammer the seats out, which will close up the valve lash and leave your valves hanging open...that is if you don't seize a valve to the guides first!
Remember what year your engine was built and how fatigued the valve train components are. Henry may have built them right with the technology he had available then, but this is now and we have the modern components you need.
These Flathead Ford valve guides are heat treated to be compatible with today's low-lead fuels. Fits 1932-53 engines. If using in a 1932-48 engine, you will have to use 1949-53 style retainers, keepers and valves.
Flathead Valve Assembly (Link)