The flathead Ford V8 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 0.060 oversize valve seats are made of a specially hardened material to combat the effect of unleaded fuel and higher performance. Machine work is required for installation. Intake has a 30° seat and exhaust has a 45° seat.
Flathead Valve Assembly (Link)