[olddog]

I think the 4V Clevland was made from 69 to 73 give or take a year. Unleaded fuel was required in the 1975 model year cars. Additionally the 4V 351C were all high compression engines. When the 87 octane unleaded hit, the compression was dropped to 8.5:1. I do not think any 4V heads would have been built to run unleaded. So I doubt any had hardened seats.

I also fuzzily remember that the manufactures did not realize that the unleaded fuel was going to eat the seats up until after the public found out the hard way. Seems like this was't discovered until the cars were on the road about a year. But I don't remember this clearly, so it could be wrong.