Hardened valve seats - do they really matter?

In a light car like an A body that will haul around only human loads, IMO, they aren't necessary. If you were talking about even a shop truck, I would say yeah. Even a crappy old shop truck hauls or pulls something every now and then. Exhaust temperature is what it's all about. In a light car that's only used for transportation, exhaust temperatures are low and the need for hardened seats is just not there.