You dont need hardend seats for unleaded gas. Hardened seats didnt come out until 1975. There are still millions of cars running without hardend seats and using unleaded gas. Unless your trying to run a 5000 pound motorhome uphill on 85 octane you should be fine. For street or mild race there is nothing wrong with stock 68-75 head seats. I have a 302 and 351 car both pre 72 with over 130,000 miles running on unleaded gas just fine.
All valve seats are (were) surface hardened during the manufacturing process. Once you grind the seats you grind away the hardened surface and the seats should be replaced with hard seats to make them last.
This guy a member on my local mustang board. Give him a shout and see if he still has them for sale. Worth the drive IMO http://phoenix.craigslist.org/nph/pts/1743547725.html
First you have to remember that in 72 although the muscle car era was ending, muscle cars were only a few years old. The problem with non hardened seats was never unleaded gas. It was something called valve recession being caused from higher than normal spring pressures for cams being used in the muscle age on non hardened seats. The seats will actually smash down over time because the iron was not as hard as it should be for higher spring pressures. Not all cars had problems, and not all motors had issues, just as not all motors had the same spring pressures. Common sence tells you that if it was the gas then all motors with non-hardened seats would of been effected. Its more directly related to the spring pressures. This is why if you talking about a stock 302 which has a mere .380 lift camshaft your going to be ok. If your talking about a stock 429 with a .450 lift cam and 140 pounds at the seat spring pressure then you may need hardened seats, but not for unleaded gas, its to keep from having valve recession. Unleaded or leaded gas would not have made a difference. Unleaded gas has always been blamed as it seemed to be the only change at the time, it certainely could not have been the Big 3 car makers with a design flaw.
