maybe somebody can me help me here. i built a 350 a while back and put a set of 993 heads on it from 1972. these were the original heads that came with the motor. i didn't think of it at the time, but a few resources i've come across say the 72 versions do not have hardened seats for unleaded. i've drove the vehicle for about a year and haven't had any problems. but i don't know what one does when you run unleaded in an old style head. and i don't know if they were changed over in some point of its life. so anyway, what does it do if you run unleaded gas in a leaded gas head? thanks if anyone can help me.