The machinist that I use gives the same advice as Jerry about not installing hardened valve seats. He will install them, but only if the seats can't be ground or if the customer insists. He compares installing the hardened seats to sleeving a cylinder...don't do it if you don't have to.

Pre68 Dave has a good article about this on his website. I use his advice and use stainless exhaust valves instead of installing hardened seats.

http://chev235guy.blogspot.com/search/label/Hard%20Seats

Others will disagree and will argue that hardened seats are the only way to go.

Matt