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.
I disagree. If you have oil anywhere near your exhaust valve seats, you have other problems.
I would recommend it. My 351 heads did not have them, and when I took the motor apart last summer, the exhaust valves in some cylinders were recessed far enough into the head to go through the seat area.
For local, low rpm driving, you can get by without it, but if you do any road trips ar extended rpm driving, you will want hardened seats. The wear increases exponentially with rpm and duration.
If you are familier with what fresh heads look like, the valve sits high enough in the seat to sit above the combustion chamber ceiling. As you can see in the photo, the exhaust valve has receeded to the point where it is the thickness of a 50 cent piece below it. Not good...