what thickness should the hardened valve seats be? I bought Engine Builder 1.7 + 1.44 valves. He suggests 1.5 OD x 3/32 thick seats. The guy installing them thinks that seems thin. He usually uses 1/4 thick, but this is his first slant 6. I don't want to run into water jackets. How much metal is there to work with? What have other people used for seat sizes?

Use the thin ones. There is not much meat there. That's what my machinist used. Keep them small diameter too, I can't remember the dimension but it's a little smaller diameter than the exhaust valve is. I think 1 9/16 OD.

Harden seats for exhaust valves only.
Use the thin 1 7/16 OD if your machinest has a valve "milling" machine to accuratly locate and install the seats.
DD