What year 340 heads got hardened seats?

71 j heads is the earliest I remember with harden ex seats.

I did sink the ex valves in the 318 in my 68, 3/4 ton. Pulled a lot of goose neck trailers with it. It's in my barn, with a m880, 360 in it now. I wore the 318 out at about 350,000 miles. It now has over 500,000 miles on it. That truck has been everywhere from the Big Apple Circus at the Lincoln Center in New York City to Portland Oregon to deliver horses. From Juarez Mexico to Montreal Canada.