There's is no need to put hardened seats in a flathead. In fact most flatheads already came with hardened exhaust valve seats through mid 1951.
If you need to buy new valves, Manley SS 1.5" or 1.6" Chevy exhaust valves are a good choice, especially if you are using a re-ground cam because they are longer than stock.
When I was an apprentice mechanic (1957) I ground flathead valves daily, then finishing the stems (to adjust them) Nice, those old hollow Ford tappets. No need for further adjustment! We got some long stem valves in '59, (might have been Chev) I did my roadster up with a Potvin cammed flattie with non-adjustable Ford tappets, the old way. All the 'rodders' told me it "wouldn't work without adjustable tappets, ya GOTTA have 'em!" I laughed, my usual chuckle.
I just did a valve job on an 8BA flathead ('49 - '53). Just use the modern stainless valves, guides and keepers. Replacing seats requires machine shop work. If they will clean up with a light grinding and have a good margin, keep them. They are pretty tough critters. Early valves with the mushroom ends can cause a lot of headaches if you have severely stuck guides. You didn't say what year your engine is but the modern valves and parts fit Ford flathead engines of many years. Install adjustable lifters if you don't have them.