We Canadians have not had leaded fuel since 1986, and I have seen no increase in valve failures due to unleaded fuel in my business. In fact, unleaded fuel creates far less carbon deposit in the combustion chamber, and in street driven engines contributes to longer valve lift.
Recession is caused by continuous operation at high power and RPM settings.
What you need is a good quality valve grind which establishes the correct seat to face contact pattern. It is quite narrow on these engines, and if done properly will last a long time. I have never used hardened seats, as they can cause heat transfer problems when poorly installed, although they are very hard and resist wear. And I second the opinion that octane has nothing to do with valve problems. I'm trying to visualize what the problems, as I'm having this done to an E head for a B20 now.