If you're replacing valves, and stainless steel exhaust valves are available, it's a good idea to go with the newer alloy. Unless you need to replace the valve seats anyway, it's not really necessary to go to the stellite seats. Stellite is really nothing new for valve seats- - - -Propane-fuel engines have needed them for decades. The idea that leaded fuel somehow lubricated valve seats has sort of been debunked, but I have seen examples of engines that were switched from leaded to unleaded gas that had problems with exhaust valves receeding into the head. I think it was more of a situation of abuse and/or heavy loading, than actual fuel-related problems.
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!