The only time hardened seats is an issue is if you drive your car 15-20K miles a year or more in hot weather w/ a hot thermostat. As little as we drive our Camaros, it's not a concern. Lead is a lubricant, which in our old cars helps to maintain the valves and valve seats over a long period of time. With today's fuel, you can control many of the issues by following certain guidelines. Run a cooler thermostat or no thermostat to keep the water temp down. This helps a lot. Run a colder spark plug. That keeps the combustion chambers cooler so the engine will not detonate. If the engine is running cooler, there is less chance for seat degredation. Couple this with running the carb a step or two richer, shortening the distributor curve, etc. All of these things help a lot. There is too much to go into trying to explain how to deal with today's fuels as I'd be writing all night. This has been an on-going learning curve that I've had to deal with over the past 20 years or so.
Over the years, we have had many Z28 owners come to us over the same issues with their cars, pinging, no power, engine run on, etc. In all cases, we were able to solve these problems by following most of the parameters that I've mentioned here, and some others. For example, running AC 45 plugs with today's gas in a stock 302, w/180 degree thermostat and factory jetting during the summer months will have your Z28 engine sounding like a marble machine. Just by taking the thermostat down to a 160, adding AC44 plugs, and stepping up the jetting makes a world of difference. Just ask any of our Z28 customers.
One last thing to mention, I do highly recommend adding some leaded Av Gas, Cam II, VP, etc. to your fuel tank if your car is going to sit for a long period of time. The unleaded fuels are harder on carb parts than anything else if the car sits for a long period of time...........six weeks or longer.
Hope this info is useful to all on this site. Call me during shop hours if you have any questions about these issues. Be glad to help.