Ditto to all the above comments. The only engines I've found that absolutely need hard seats are the ones using "dry" fuels- - - -Propane or compressed natural gas. For some reason, those fuels can cause the exhaust valve seats to erode and the valves to sink deeper into the head. If someone who is rebuilding a stovebolt head wants a little extra protection, replacing the exhaust valves with stainless steel ones for a small block Chevy V8 is a lot less hassle and expense compared to replacing the valve seats. They're a drop-in fit- - - -no modifications but a good 3-angle valve seat job needed, with an.090" valve to seat contact patch.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!