I would recommend that you have the hardened valve seats installed before you drop the engine in. It would definitely increase the lifespan of those heads. I rebuilt the engine in my 'stang 20 years back and I did not bother installing the hardened valve seats. I guess I thought they were bluffing with that phase out of lead from our gas. Anyway, I have not had a problem with my engine, but it has always been a weekend driver. Hey, if you have the opportunity and you plan on driving it regularly, then definitely go for the hardened valve seats.