you mentioned hardened seats... first of all- the book says 346 heads have hardened seats (for what that is worth), second of all- I've always been skeptical of the need for hardened seats, third- installing hardened seats makes me nervous for fear of dropping the seat and the calamity that would ensue.

My little anecdote on the subject- I've put tens of thousands of miles on 60's headed engines without hardened seats using straight unleaded gas, including one engine that I put around 40k on top of the 110k that was on it when I bought it. I'm presuming that for much of its life before I got the car it was also deprived of leaded gas or additives. When I went to rebuild it, the seats didn't have any abnormal wear compared to any other 150k big block head and from then on I quit worrying about it. My point is just that from my experiences the risk of damage from running unleaded gas without having hardened seats doesn't justify the cost and risk of installing hardened seats. Feel free to apply your own judgment.