Ok I'm finaly going to add my 2cent worth to this topic.
First Off There is a very good Risk to hitting water when going to install a Hardened seat.
#1 don't DO is have a Deep seat installed at ALL This will pretty much Guarantee your going to be Hitting Water.#2 Use a Shallow seat this will lesson the problem But still No Guarantee That you won't hit water.
When ever Your going to a Bigger Valve This becomes a ISSUE with Hardened Seats.AND The Biggest REASON for this is CORE SHIFT in the head casting.
So with SOoooooooooo Many lead additives Out there
Why Run the Risk if your that worried about Hardened seats USE IT. Why Run the risk of having a Head That has been CUT Right into the water jacket NOW your Back to Square ONE.
I ran My 64 FOR the Most Part which was at the Time a Daily driver/Racer To nothing But a FULL time Racer Nearing the end of 17 Plus years AND Never seen Or had a Issue with Out running Hardened seats.And Fuel octange ranging from LCheap 87 to 112 On the track.
So like i said My 2Cent oppion.
God doesn't like ugly.
