Again: Unleaded or leaded fuel for mid 70's /6?

I dont think the slants ever got hardened seats. The seats were cut into the iron head and induction hardened. Not quite as good as a real hard seat but better than nothing.
75 was the year mopar went to unleaded fuel and catalytic converters. The instrument cluster would have said "unleaded fuel only". In the early years they just slapped a sticker on it.
Octane has nothing to do with the need for hardened seats. The need came when they removed the lead from fuel since lead would do bad things to a catalytic converter. If your concerned try to find a lead substitute to add to your fuel.