Hardened valve seats - do they really matter?

Princess Valiant said
So i am about to invest a couple hundred bucks and mill a slant six head .100

However I remember in auto school being told that the 73 and up heads have hardened valve seats to run better on the then new unleaded fuel.

I can say I have never run a lead additive on the 73 and older heads and I've never had any issues.

Before I put money into machine work on a head like that, should I try to find a 73 or later head to mill, or just mill my 68 head and don't worry about it?

For me it would be determined if you were having a valve job done. If so, then I would install them, they are already machining. I you just planned to lap them, then I wouldn't worry about it. Paying for milling - Don't you have a mill? C'mon girl. You have high temps and elevation by you, make sure you are running rich enough.