What year did Chrysler start building engines with hardened valve seats for unleaded gas? Would 1969 440 HP engines have them?

Pretty sure that year did not . . . think it started with '73model year for all engines??? Maybe the 360 in '70 ??? Am sure someone with more knowledge than me will chime in . . .

That should have the '906 heads with un-hardened seats.

1973 452 heads for big blocks.
factory heads for 1969 (906) will not have factory hardened valve seats.
You can either rebuild your 906's and install hardened valve seats,or
rebuild a set of 452's which flow equally as well as the original 906's.
The 452 upgrade has been a popular swap for many years.

Cost of re-doing big block heads nowadays almmost warrants buying new aluminum cylinder heads and install striaght out of the box,IMHO

On BB as far I know, 346s are the first ones, but 452 were popular because were hardened using a more efficient method. i can be wrong, but that's what I have read.

I also believe that the 346 heads were the first.
The factory used induction hardening which is a surface hardening process. They did not use inserted hardened seats. A lot of people have said that after grinding the seats the first time, the factory hardening is gone.

Weren't the 346 castings a one year only, 1971?
I was thinking the 916 where used in 1972 and maybe up to 1974 or 75 in passenger cars

346 were up to 73/74... 452 since 75/76