What fuel is best?

I tell you man, I've built a lot of engines, but I've torn down probably three or four times as many. I've seen valve seat recession from unleaded gas on non hardened seats, but not on a large scale. IMO, they are slap not needed for a lighter car like an A body. Now, if you had a truck you were using for towing and hauling on a regular basis, then yes, I would recommend them, but IMO they are a colossal waste of money on something like a light hot rod, or even a light stock vehicle. The exhaust gas temperatures are just not hot enough. I see nothing wrong with t good lead substitute if that's what you want to run.